Semiconductor memory

ABSTRACT

A partial area for retaining data during low power consumption mode is composed of a single first memory cell out of a plurality of memory cells connected to a bit line. An operation control circuit operates any of the memory cells selected in accordance with an address signal during normal operation mode for performing a read operation and a write operation. The operation control circuit keeps latching data retained by the first memory cell in the partial area into a sense amplifier during the low power consumption mode. This eliminates the need for a refresh operation for retaining the data in the first memory cell during the low power consumption mode. Since the data can be retained without a refresh operation, it is possible to reduce the power consumption during the low power consumption mode.

CROSS-REFERRENCE TO RELATED APPLICATIONS

This is a Divisional Application, which claims the benefit of U.S.patent application Ser. No. 10/968,811 filed Oct. 20, 2004, now U.S.Pat. No. 7,209,402 which in turn is a Divisional Application of parentapplication Ser. No. 10/335,949, filed Jan. 3, 2003, now U.S. Pat. No.6,829,192 B2 issued Dec. 7, 2004. The Disclosure of the priorapplication is hereby incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory which requiresfresh operations to retain data written in its memory cells.

2. Description of the Related Art

Hand-held terminals such as cellular phones are growing in memorycapacity requirement year by year. Under the circumstances, dynamic RAMs(hereinafter, referred to as DRAMs) have come to be used as the workmemories of the cellular phones instead of conventional static RAMs(hereinafter, referred to as SRAMs). DRAMs are smaller than SRAMs in thenumbers of devices that constitute the memory cells. DRAMs can thus bereduced in chip size, with lower chip cost than that of SRAMs.

Meanwhile, semiconductor memories to be mounted on cellular phones mustbe low in power consumption so as to allow prolonged use of thebatteries. Unlike SRAMs, DRAMs require periodic refresh operations inorder to retain data written in their memory cells. Consequently, whenDRAMs are used as the work memories of cellular phones, data retentionalone can consume power to exhaust the batteries even if the cellularphones are not in use.

In order to reduce the power consumption of the DRAMs during standby (inlow power consumption mode), there have been developed partial refreshtechnology and twin cell technology.

According to the partial refresh technology; the number of memory cellsto be refreshed is reduced by limiting the number of memory cells toretain data in a standby state. Reducing the memory cells to refresh candecrease the number of times of refresh, with a reduction in the powerconsumption during standby.

According to the twin cell technology, complementary data is stored intotwo memory cells (memory cell pair) which are connected to complementarybit lines, respectively. This doubles the charges retained in the memorycell pair. Since the two memory cells retain “H” data and “L” data,respectively, the refresh interval is determined by a longer one betweenthe data retention times of “H” data and “L” data. That is, the worstdata retention time is the sum of the characteristics of the two memorycells, not the characteristic of one single memory cell. On thecontrary, in a single memory cell, the refresh interval is determined bya shorter one between the data retention times of “H” data and “L” data.As above, according to the twin cell technology, retaining data in twomemory cells makes it possible to compensate a small leak path, if any,in one of the memory cells with the other memory cell.

In the partial refresh technology described above, to reduce the powerconsumption during the low power consumption mode requires that the dataretention areas be small. As a result, the lower the power consumption,the smaller the memory capacity available for retention during the lowpower consumption mode.

In the twin cell technology, two memory cells are always used to retaina single bit of data not only in refresh operations but also in normalread operations and write operations. Storing a single bit hencerequires a memory cell size twice as big as that of a single memorycell, which results in increasing chip cost. Consequently, in the casesof DRAMs to which the twin cell technology is applied, there is not muchadvantage in replacing the SRAMs mounted on cellular phones with theDRAMs.

SUMMARY OF THE INVENTION

It is an object of the present invention to reduce the power consumptionof a semiconductor memory in low power consumption mode, thesemiconductor memory having memory cells that require refreshoperations.

According to one of the aspects of the semiconductor memory of thepresent invention, a partial area for retaining data in low powerconsumption mode is composed of a single first memory cell out of aplurality of memory cells connected to a bit line. An operation controlcircuit operates any of the memory cells selected in accordance with anaddress signal during normal operation mode for performing a readoperation and a write operation. The operation control circuit keepslatching data retained by the first memory cell in the partial area intoa sense amplifier during low power consumption mode. This eliminates theneed for a refresh operation for retaining the data in the memory cellduring the low power consumption mode. Since the data can be retainedwithout a refresh operation, it is possible to reduce the powerconsumption in the low power consumption mode.

According to another aspect of the semiconductor memory of the presentinvention, a word line control circuit of the operation control circuitenables selection of a partial word line of word lines connected to thememory cells and disables selection of the other word lines, the partialword line being connected to the memory cell in the partial area. Asense amplifier control circuit of the operation control circuit keepsactivating the sense amplifier during the low power consumption mode.Since the selection of the word lines other than in the partial area isdisabled during the low power consumption mode, the sense amplifierkeeps latching the data that is read from the memory cell. Consequently,data crash can be avoided during the low power consumption mode.

According to another aspect of the semiconductor memory of the presentinvention, the word line control circuit keeps selecting the partialword line during the low power consumption mode while the senseamplifier keeps latching the data. This simplifies theselecting/deselecting control of the word lines. That is, it is possibleto reduce the scale of the control circuit for the word lines.

According to another aspect of the semiconductor memory of the presentinvention, a booster for supplying a boost voltage to the word linesstops its operation after the sense amplifier latches data at the startof the low power consumption mode. In returning from the low powerconsumption mode to the normal operation mode, the booster starts aboost operation again. Since the booster is operated only when theselecting operation of the word lines is necessary, the powerconsumption in the low power consumption mode can be reduced further.

According to another aspect of the semiconductor memory of the presentinvention, a mask circuit disables the selection of the word lines inresponse to a refresh control signal in the low power consumption mode.The semiconductor memory is thus prevented from malfunctioning.

According to another aspect of the semiconductor memory of the presentinvention, its operation mode shifts to the normal operation mode or thelow power consumption mode in accordance with a chip enable signal foroperating the semiconductor memory. Thus, shifting of the operation modeof the semiconductor memory can be made by simple control. This enablesa simple configuration of the control circuit of a system implementingthe semiconductor memory.

According to another aspect of the semiconductor memory of the presentinvention, the operation control circuit selects first and second wordlines simultaneously in second and subsequent refresh operations on eachof the partial areas in the low power consumption mode. The operationcontrol circuit can thus be configured simply.

According to another aspect of the semiconductor memory of the presentinvention, a plurality of partial areas for retaining data during lowpower consumption mode are each composed of a predetermined number ofmemory cells, of memory cells connected to a bit line. A refresh controlcircuit cyclically outputs a refresh control signal for refreshing thememory cells. An operation control circuit performs a read operation, awrite operation, and a refresh operation on the memory cells. Thepartial areas each include a single first memory cell and at least asingle second memory cell which are of the memory cells connected to thebit line.

At the start of the low power consumption mode, the operation controlcircuit performs a refresh operation on data retained in the firstmemory cell. The data is amplified by a sense amplifier and written tothe first and second memory cells in the refresh operation.Consequently, the data in the first memory cell can be written to thesecond memory cell(s) with reliability. The operation control circuitsubsequently refreshes the first and second memory cells simultaneouslyin response to the refresh control signal during the low powerconsumption mode. Since data retained in a single memory cell isretained by using a plurality of memory cells during the low powerconsumption mode, it is possible to extend the retention time over whichthe data can be retained. Consequently, the refresh intervals can bemade longer in the low power consumption mode than in normal operations.A reduction in the frequency of refresh operations can reduce the powerconsumption in the low power consumption mode.

According to another aspect of the semiconductor memory of the presentinvention, in each of the partial areas, the first memory cell isconnected to a first word line and the second memory cell(s) is/areconnected to a second word line(s). A word line control circuit of theoperation control circuit starts selection of the first word lineearlier than selection of the second word line(s) in a first refreshoperation on each of the partial areas in the low power consumptionmode. This can prevent the data in the second memory cell(s) from beingread first to destroy data retained in the first memory cell. That is,the semiconductor memory can be prevented from malfunctioning.

According to another aspect of the semiconductor memory of the presentinvention, the refresh control circuit outputs, in second and subsequentrefresh operations in the low power consumption mode, the refreshcontrol signal at intervals longer than in the normal operation mode.This can lower the refresh frequency in the low power consumption modeand reduce the power consumption.

According to another aspect of the semiconductor memory of the presentinvention, the refresh control circuit performs, in shifting from thelow power consumption mode to the normal operation mode, a refreshoperation only on the memory cell(s) on which a predetermined timeelapses after a previous refresh operation has been performed.Performing a refresh operation on the necessary memory cell(s) allows aquick shifting from the low power consumption mode to the normaloperation mode. Returning to the normal operation mode quickly canimprove the operation efficiency of a system on which the semiconductormemory is mounted.

According to another aspect of the semiconductor memory of the presentinvention, the refresh control circuit outputs, in shifting from the lowpower consumption mode to the normal operation mode, the refresh controlsignal at intervals shorter than in the normal operation mode. Thisallows quick return from the low power consumption mode with theimproved operation efficiency of a system on which the semiconductormemory is mounted.

According to another aspect of the semiconductor memory of the presentinvention, a switch circuit divides a bit line into first and second bitlines. A partial area is composed of a first memory cell out of memorycells, the first memory cell being connected to a first bit line lyingon a side of the switch circuit closer to a sense amplifier. A refreshcontrol circuit cyclically outputs a refresh control signal forrefreshing the memory cells. A switch control circuit turns on theswitch circuit in the normal operation mode, and turns off the same inthe low power consumption mode. Since the bit line connected to thesense amplifier decreases in bit line capacity during the low powerconsumption mode, the sense amplifier can surely latch data retained inthe first memory cell even if the signal quantity of the data is low. Asa result, it is possible to lower the refresh frequency during the lowpower consumption mode, resulting in reducing the power consumption.

According to another aspect of the semiconductor memory of the presentinvention, a plurality of word lines to be selected in accordance withan address signal are connected to the memory cells, respectively. Aword line control circuit selects any of the word lines in accordancewith the address signal during the normal operation mode. The word linecontrol circuit enables selection of a partial word line and disablesselection of the other word lines during the low power consumption mode,the partial word line being one of the word lines and connected to thefirst memory cell in the partial area. Since the selection of the wordlines other than in the partial area is disabled during the low powerconsumption mode, the sense amplifier keeps latching the data that isread from the memory cell through the selection of the partial wordline. Consequently, data crash can be avoided during the low powerconsumption mode.

According to another aspect of the semiconductor memory of the presentinvention, first and second memory cells are connected to complementarybit lines, respectively. A sense amplifier is connected to thecomplementary bit lines. A refresh control circuit cyclically outputs arefresh control signal for refreshing the first and second memory cells.An operation control circuit operates either of the first and secondmemory cells selected in accordance with an address signal during normaloperation mode for performing a read operation and a write operation.

At the start of low power consumption mode, the operation controlcircuit makes the sense amplifier amplify data retained in the firstmemory cell, and writes it to the first and second memory cells (refreshoperation). Consequently, the data in the first memory cell can bewritten to the second memory cell(s) with reliability. The operationcontrol circuit subsequently refreshes the first and second memory cellssimultaneously in response to the refresh control signal. The operationcontrol circuit subsequently refreshes the first and second memory cellssimultaneously in response to the refresh control signal during the lowpower consumption mode. Since data retained in a single memory cell isretained by using a plurality of memory cells during the low powerconsumption mode, it is possible to extend the retention time over whichthe data can be retained. Consequently, the refresh intervals can bemade longer in the low power consumption mode than in normal operations.The lower frequency of refresh operations can reduce the powerconsumption in the low power consumption mode.

According to another aspect of the semiconductor memory of the presentinvention, the first memory cell is connected to a first word line andthe second memory cell is connected to a second word line. The operationcontrol circuit starts selection of the first word line earlier thanselection of the second word line in a first refresh operation in thelow power consumption mode. This can prevent the data in the secondmemory cell from being read first to destroy data retained in the firstmemory cell. That is, the semiconductor memory can be prevented frommalfunctioning.

According to another aspect of the semiconductor memory of the presentinvention, the first and second word lines are adjacent to each other.This simplifies the circuit layout of a decoder and the like forselecting the first and second word lines.

According to another aspect of the semiconductor memory of the presentinvention, its operation mode shifts to the normal operation mode or thelow power consumption mode in accordance with a command supplied througha command terminal. Thus, the operation mode of the semiconductor memorycan be shifted by simple control. As a result, the control circuit of asystem implementing the semiconductor memory can be configured simply.

According to another aspect of the semiconductor memory of the presentinvention, a first memory cell and a plurality of second memory cellsare each connected to either of complementary bit lines. A senseamplifier is connected to the complementary bit lines. A refresh controlcircuit cyclically outputs a refresh control signal for refreshing thefirst and second memory cells.

An operation control circuit operates any of the first and second memorycells selected in accordance with an address signal during normaloperation mode for performing a read operation and a write operation.The operation control circuit performs at the start of low powerconsumption mode a refresh operation in which data retained in the firstmemory cell is amplified by the sense amplifier and written to the firstand second memory cells, and subsequently refreshes the first and secondmemory cells simultaneously in response to the refresh control signal.Since data retained in the single memory cell is retained by using thefirst memory cell and the plurality of second memory cells during thelow power consumption mode, it is possible to extend the retention timeover which the data can be retained. Consequently, the frequency ofrefresh operations can be further reduced for a significant reduction inthe power consumption during the low power consumption mode.

According to another aspect of the semiconductor memory of the presentinvention, the operation control circuit selects second word lines insuccession after the selection of a first word line. It is thereforepossible to disperse the consumption current of the control circuit thatoperates to select the word lines. This can reduce power supply noiseand the like that occur with the selection of word lines.

According to another aspect of the semiconductor memory of the presentinvention, first and second memory cells are connected to complementarybit lines, respectively. A sense amplifier is connected to thecomplementary bit lines. For operation mode, the semiconductor memoryhas a first operation mode, a second operation mode, and a thirdoperation mode.

In the first operation mode, at least either of a read operation and awrite operation is performed on the first and second memory cells. Inthe second operation mode, data retained in the first memory cell islatched into a sense amplifier, and the latched data and inverted datathereof are written to the first and second memory cells, respectively.In the third operation mode, the data retained in the first memory celland the inverted data retained in the second memory cell are latchedinto the sense amplifier, and the latched data and inverted data thereofare written to the first and second memory cells, respectively.

Since data retained in a single memory cell is retained by using aplurality of memory cells during the second operation mode, theretention time over which the data can be retained becomes longer in thethird operation mode. Consequently, the frequency of data rewrite duringthe third operation mode decreases, which allows a reduction in thepower consumption during low power consumption mode.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature, principle, and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings in which like parts aredesignated by identical reference numbers, in which:

FIG. 1 is a block diagram showing a first embodiment of thesemiconductor memory of the present invention;

FIG. 2 is a circuit diagram showing the details of the PA controlcircuit and the word line control circuit shown in FIG. 1;

FIG. 3 is a circuit diagram showing the details of the word decodershown in FIG. 1;

FIG. 4 is a block diagram showing the details of essential parts of thememory core shown in FIG. 1;

FIG. 5 is a timing chart showing the operation of the pseudo SRAMaccording to the first embodiment;

FIG. 6 is a block diagram showing a second embodiment of thesemiconductor memory of the present invention;

FIG. 7 is a circuit diagram showing the details of the PA controlcircuit and the word line control circuit shown in FIG. 6;

FIG. 8 is a timing chart showing the operation of the pseudo SRAMaccording to the second embodiment;

FIG. 9 is a block diagram showing a third embodiment of thesemiconductor memory of the present invention;

FIG. 10 is a circuit diagram showing the details of the refresh timershown in FIG. 9;

FIG. 11 is a circuit diagram showing the details of the refresh registershown in FIG. 9;

FIG. 12 is a circuit diagram showing the details of the refresh registershown in FIG. 9;

FIG. 13 is a timing chart showing the operation of the refresh timer andthe refresh register;

FIG. 14 is a circuit diagram showing the details of the word linecontrol circuit shown in FIG. 9;

FIG. 15 is a circuit diagram showing the details of the word decodershown in FIG. 9;

FIG. 16 is a block diagram showing the details of essential parts of thememory core shown in FIG. 9;

FIG. 17 is a timing chart showing the refresh operations of the pseudoSRAM according to the third embodiment;

FIG. 18 is a timing chart showing the operation of the pseudo SRAMaccording to the third embodiment;

FIG. 19 is a block diagram showing a fourth embodiment of thesemiconductor memory of the present invention;

FIG. 20 is a circuit diagram showing the details of the refresh timershown in FIG. 19;

FIG. 21 is a timing chart showing the operation of the pseudo SRAMaccording to the fourth embodiment;

FIG. 22 is a block diagram showing a fifth embodiment of thesemiconductor memory of the present invention;

FIG. 23 is a circuit diagram showing the details of the refresh timershown in FIG. 22;

FIG. 24 is a block diagram showing the details of essential parts of thememory core shown in FIG. 22;

FIG. 25 is a block diagram showing the details of essential parts of thememory core according to a sixth embodiment of the semiconductor memoryof the present invention;

FIG. 26 is a circuit diagram showing the details of the sense amplifiersand column switches shown in FIG. 25;

FIG. 27 is a timing chart showing the operation of the pseudo SRAMaccording to the sixth embodiment;

FIG. 28 is a block diagram showing a seventh embodiment of thesemiconductor memory of the present invention;

FIG. 29 is a block diagram showing the details of the operation modecontrol circuit shown in FIG. 28;

FIG. 30 is a timing chart showing the operation of the operation modecontrol circuit shown in FIG. 28;

FIG. 31 is a block diagram showing the details of the refresh timershown in FIG. 28;

FIG. 32 is a timing chart showing the operation of the refresh timer andthe refresh command generator according to the seventh embodiment;

FIG. 33 is a block diagram showing the details of the refresh addresscounter shown in FIG. 28;

FIG. 34 is a timing chart showing the operation of the resetting circuitshown in FIG. 33;

FIG. 35 is an explanatory diagram showing the operation of the refreshaddress counter shown in FIG. 33;

FIG. 36 is a block diagram showing the details of essential parts of thememory core shown in FIG. 28;

FIG. 37 is a circuit diagram showing the details of the ¼ word decodershown in FIG. 36;

FIG. 38 is a circuit diagram showing the details of the sense amplifiersand precharge circuits shown in FIG. 36;

FIG. 39 is a timing chart showing the operation of the sense amplifiercontrol circuit and the precharge control circuit shown in FIG. 28;

FIG. 40 is a timing chart showing the operation of the seventhembodiment in normal operation mode;

FIG. 41 is a timing chart showing the operation of the seventhembodiment in common refresh mode;

FIG. 42 is a timing chart showing the operation of the seventhembodiment in partial refresh mode and concentrated refresh mode;

FIG. 43 is a timing chart showing the operation of the pseudo SRAMaccording to the seventh embodiment;

FIG. 44 is a block diagram showing an eighth embodiment of thesemiconductor memory of the present invention;

FIG. 45 is a block diagram showing the details of the operation modecontrol circuit shown in FIG. 44;

FIG. 46 is a timing chart showing the operation of the operation modecontrol circuit shown in FIG. 44;

FIG. 47 is a block diagram showing the details of the refresh timershown in FIG. 44;

FIG. 48 is a timing chart showing the operation of the refresh timer andthe refresh command generator according to the eighth embodiment;

FIG. 49 is a block diagram showing the details of the refresh addresscounter shown in FIG. 44;

FIG. 50 is an explanatory diagram showing the operation of the refreshaddress counter shown in FIG. 49;

FIG. 51 is a block diagram showing the details of essential parts of thememory core shown in FIG. 44;

FIG. 52 is a circuit diagram showing the details of the ¼ word decodershown in FIG. 51;

FIG. 53 is a timing chart showing the operation of the sense amplifiercontrol circuit and the precharge control circuit shown in FIG. 44;

FIG. 54 is a timing chart showing the operation of the eighth embodimentin normal operation mode;

FIG. 55 is a timing chart showing the operation of the eighth embodimentin common refresh mode;

FIG. 56 is a timing chart showing the operation of the eighth embodimentin partial refresh mode and concentrated refresh mode;

FIG. 57 is a block diagram showing another example of the memory coreaccording to the fifth embodiment;

FIG. 58 is a timing chart showing another example of operation of theseventh embodiment in common refresh mode; and

FIG. 59 is a timing chart showing another example of operation of theeighth embodiment in partial refresh mode and concentrated refresh mode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings. In the drawings, each thick line represents asignal line that consists of a plurality of lines. Signals ending in “Z”are of positive logic. Signals with a leading “/” and signals ending in“X” are of negative logic. Double circles in the drawings representexternal terminals. In the following description, signal names may beabbreviated like a “CLK signal” for a “clock signal CLK” and a “CEsignal” for a “chip enable signal CE”.

FIG. 1 shows a first embodiment of the semiconductor memory of thepresent invention. This semiconductor memory is formed as a pseudo SRAM,which has DRAM memory cells and an SRAM interface, by using CMOStechnology. The pseudo SRAM performs refresh operations within the chipat regular time intervals without receiving a refresh command fromexterior, thereby retaining data written in its memory cells. Thispseudo SRAM is used as a work memory to be mounted on a cellular phone,for example.

The pseudo SRAM includes a command decoder 10, a PA control circuit 12,a mode register 14, a refresh timer 16, a refresh command generator 18,a refresh address counter 20, an address buffer 22, a data input/outputbuffer 24, a multiplexer 26, a core control circuit 28, and a memorycore 30. The refresh timer 16, the refresh command generator 18, and therefresh address counter 20 operate as a refresh control circuit forrefreshing memory cells.

The command decoder 10 receives command signals (a chip enable signalCE, a write enable signal /WE, and an output enable signal /OE) fromexterior, decodes the received commands, and outputs a read controlsignal RDZ or a write control signal WRZ. The PA control circuit 12outputs the inverted logic of the chip enable signal CE as a partialsignal PAZ while receiving a mode signal PAMDZ of high level.

The mode register 14 outputs the mode signal PAMDZ of high level whenpartial mode to be described later (a kind of low power consumptionmode) is established by a mode register setting command. When the moderegister 14 is set at normal standby mode (another kind of the low powerconsumption mode), it outputs the mode signal PAMDZ of low level. Thepseudo SRAM recognizes the supply of the mode register setting command,for example, when it receives a predetermined combination of commandsignals a plurality of times. Then, the mode register 14 is set inaccordance with the logical value of the data signal supplied to a dataterminal DQ at that time.

The refresh timer 16 outputs a refresh request signal TREF (refreshcommand) at predetermined cycles. The refresh command generator 18outputs a refresh control signal REFZ in synchronization with therefresh request signal TREF when it receives the refresh request signalTREF in advance of the read control signal RDZ or the write controlsignal WRZ. When the refresh command generator 18 receives the refreshrequest signal TREF after the read control signal RDZ or the writecontrol signal WRZ, it outputs the refresh control signal REFZ after aread operation corresponding to the RDZ signal or a write operationcorresponding to the WRZ signal. That is, the refresh command generator18 operates as an arbiter for setting priorities between read/writeoperations and a refresh operation.

Although not shown in particular, the operation of the refresh timer 16may be suspended while a partial signal is at high level. This allows areduction in the power consumption during the partial mode to bedescribed later.

The refresh address counter 20 counts in synchronization with the risingedge of the refresh control signal REFZ, thereby updating a refreshaddress REFAD.

The address buffer 22 receives an address signal AD through an addressterminal, and outputs the received signal as a row address signal RAD(upper address) and a column address signal CAD (lower address). Thatis, this pseudo SRAM is a memory of address non-multiplex type whichreceives the upper address and the lower address at the same time.

The data input/output buffer 24 receives read data through a common databus CDB and outputs the received data to the data terminal DQ. The datainput/output buffer 20 receives write data through the data terminal DQand outputs the received data to the common data bus CDB. The number ofbits of the data terminal DQ is 16 bits, for example.

The multiplexer 26 outputs the refresh address signal REFAD as a rowaddress signal RAD2 when the refresh control signal REFZ is at highlevel. When the refresh control signal REFZ is at low level, themultiplexer 26 outputs the row address signal RAD as the row addresssignal RAD2.

The core control circuit 28 includes a sense amplifier control circuit32, a word line control circuit 34, and a not-shown precharge controlcircuit. The sense amplifier control circuit 32 outputs a senseamplifier activating signal LEZ for activating sense amplifiers SA to bedescribed later when it receives any of the RDZ signal, the WRZ signal,and the REFZ signal, or when it receives the PAZ signal. The word linecontrol circuit 34 outputs a word line control signal WLZ when itreceives any of the RDZ signal, the WRZ signal, and the REFZ signal, orwhen it receives the PAZ signal. The precharge control circuit outputs aprecharging signal PREZ when the memory core 30 is not in operation. Thecore control circuit 28 operates as an operation control circuit forperforming read operations, write operations, and refresh operations.

The memory core 30 includes a memory cell array ARY, a word decoderWDEC, sense amplifiers SA, a column decoder CDEC, a sense buffer SB, anda write amplifier WA. The memory cell array ARY has a plurality ofvolatile memory cells MC (dynamic memory cells), along with a pluralityof word lines WL and a plurality of bit lines BL connected to the memorycells MC. The memory cells MC are the same as typical DRAM memory cells,each having a capacitor for retaining data in the form of a charge and atransfer transistor arranged between this capacitor and a bit line BL.The gates of the transfer transistors are connected to the word linesWL. Through the selection of the word lines WL, any of a read operation,a write operation, a refresh operation, and a partial operation to bedescribed later is performed. The memory cell array ARY performs any ofthe read operation, write operation, and refresh operation beforeexecuting a precharge operation for resetting the bit lines BL to apredetermined voltage in response to the precharging signal PREZ.

The word decoder WDEC, when receiving the word line control signal WLZof high level, selects any of the word lines WL according to the rowaddress signal RAD2 and the partial signal PAZ, and boosts the selectedword line WL to a power supply voltage. The column decoder CDEC outputs,in accordance with the column address signal CAD, a column line signal(CLZ in FIG. 4 to be seen later) for turning on column switches (CSW inFIG. 4 to be seen later) which connect the bit lines BL and the data busDB, respectively.

The sense amplifiers SA amplify the data on the bit lines BL in signalquantity. The data amplified by the sense amplifiers SA is transmittedto the data bus DB through the column switches in a read operation. In awrite operation, the data is written to the memory cells MC through thebit lines. Incidentally, as will be described later, the senseamplifiers SA are kept activated in the partial mode.

The sense buffer SB amplifies the read data on the data bus DB in signalquantity, and outputs the resultant to the common data bus CDB. Thewrite amplifier WA amplifies the write data on the common data bus CDBin signal quantity, and outputs the resultant to the data bus DB.

FIG. 2 shows the details of the PA control circuit 12 and the word linecontrol circuit 34 shown in FIG. 1.

The PA control circuit 12 has an AND circuit which is activated onreceiving the mode signal PAMDZ of high level and outputs the logiclevel of the CE signal as the partial signal PAZ.

The word line control circuit 34 has an edge detecting circuit 34 a anda NAND gate 34 b. The edge detecting circuit 34 a generates a pulsesignal of low level in synchronization with the rising edge of the RDZsignal, WRZ signal, or REFZ signal. The NAND gate 34 b receives theinverted signal of the partial signal PAZ as well as the pulse signalfrom the edge detecting circuit 34 a, and outputs the word line controlsignal WLZ. The NAND gate 34 b operates as a mask circuit for inhibitingthe word line control signal from being activated in response to theREFZ signal during the partial mode.

Specifically, the word line control circuit 34 outputs the word linecontrol signal WLZ of predetermined pulse width in synchronization withthe RDZ signal, WRZ signal, or REFZ signal when the partial signal PAZis at low level. When the partial signal PAZ is at high level, the wordline control circuit 34 keeps outputting the word line control signalWLZ of high level.

FIG. 3 shows the details of the word decoder WDEC shown in FIG. 1. Forthe sake of plain explanation, FIG. 3 shows only part of a circuit thatcorresponds to two bits of row address signals A0Z and A1Z. In fact, theword decoder WDEC receives address signals for selecting all the wordlines WL of the memory core 30.

The word decoder WDEC has NAND gates and AND circuits. The NAND gatesoutput the inverted signals of the row address signals A0Z and A1Z asaddress signals A0X and A1X, respectively, when the partial signal PAZis at low level. When the partial signal PAZ is at high level, the NANDgates fix the address signals A0X and A1X to high level. The ANDcircuits decode the address signals A0X, A1X and their inverted signalsto select any of the word lines WL (WLP, WL0, WL1, . . . ). When thepartial signal PAZ is at low level, any of the word lines WL (WLP, WL0,WL1, . . . ) is selected according to the logic of the row addresssignals A0Z and A1Z. When the partial signal PAZ is at high level, theword line WLP alone is selected regardless of the logic of the rowaddress signals A0Z and A1Z. The word line WL selected changes to highlevel.

FIG. 4 shows the details of essential parts of the memory core 30 shownin FIG. 1.

The memory cell array ARY includes the memory cells MC arranged in amatrix, the plurality of word lines WL (WL0, WL1, . . . , WLP) connectedto the memory cells MC, and the plurality of bit lines BL (BL0, BL1, . .. , BLm) connected to the memory cells MC. The memory cells MC aligningvertically in the diagram are connected to the respective same bit linesBL (any of BL0, BL1, . . . , BLm). The memory cells MC aligninghorizontally in the diagram are connected to the respective same wordlines WL (any of WL0, WL1, . . . WLP (WLn)).

The memory cells MC connected to the single word line WLP (partial wordline) constitute a partial area PA (the frame in broken thick lines).The memory cells MC in the partial area PA are connected to differentbit lines BL from one another. In this embodiment, when in the partialmode (low power consumption mode), the memory cells MC in the partialarea PA retain data while the other memory cells MC lose data.

The sense amplifiers SA are connected to the bit lines BL0, BL1, . . . ,BLm, respectively. The column switches CSW are connected to the bitlines BL0, BL1, . . . , BLm, respectively. Receiving the column linesignal CLZ of high level, the column switches CSW turn on to connect thebit lines BL and the data bus DB.

FIG. 5 shows the operation of the pseudo SRAM of the first embodiment.In this example, the mode register 14 shown in FIG. 1 is set at thepartial mode.

With reference to FIG. 5, description will be given of features of thepresent invention, or shifting from normal operation mode to the partialmode, the state during the partial mode, and shifting from the partialmode to the normal operation mode. Although not shown in particular,read operations corresponding to read commands from exterior, writeoperations corresponding to write command from exterior, and refreshoperations corresponding to refresh commands occurring internally areperformed in the normal operation mode. The read operations, writeoperations, and refresh operations in the normal operation mode areperformed the same as heretofore. Description thereof will thus beomitted here.

Initially, in the normal operation mode, the PA control circuit 12 shownin FIG. 2 changes the partial signal PAZ to high level in response tothe chip enable signal CE's changing to low level (FIG. 5( a)). The wordline control circuit 34 changes the word line control signal WLZ to highlevel in response to the partial signal PAZ of high level (FIG. 5( b)).

The core control circuit 28 shown in FIG. 1 changes the prechargingsignal PREZ to low level in response to the partial signal PAZ of highlevel (FIG. 5( c)). The change in the precharging signal PREZ releasesthe bit lines BL from the precharged state.

The word decoder WDEC shown in FIG. 3 fixes the address signals A0X andA1X to high level in response to the partial signal PAZ of high level(FIG. 5( d)). The word decoder WDEC also changes the word line signalWLP to high level in response to the address signals A0X and A1X of highlevel and the word line control signal WLZ of high level (FIG. 5( e)).That is, because of the shifting from the normal operation mode to thepartial mode, the memory cells MC in the partial area PA are selectedexclusively.

In accordance with the word line signal WLP's turning to high level, thedata retained by the memory cells MC in the partial area PA is read tothe bit lines BL (FIG. 5( f)). Subsequently, the sense amplifier controlcircuit 32 changes the sense amplifier activating signal LEZ to highlevel in response to the partial signal PAZ (FIG. 5( g)). The change inthe LEZ signal activates the sense amplifiers SA, so that the bit linesBL are amplified in signal quantity (FIG. 5( h)). Then, the senseamplifiers SA latch the data retained by the memory cells MC in thepartial area PA (FIG. 5( i)).

During the partial mode, the word line signal WLP and the senseamplifier activating signal LEZ are fixed to high level. The senseamplifiers SA, during the partial mode, thus keep latching the dataretained by the memory cells MC in the partial area PA. During thepartial mode, the internal circuitry of the pseudo SRAM maintains astatic state and makes no change in output. Since the internal circuitryis composed of CMOS circuits, the power consumption under the staticstate falls to nearly zero. Consequently, the power consumption duringthe partial mode becomes significantly smaller than in conventionalpartial mode where self refresh is performed. Next, in the partial mode,the PA control circuit 12 changes the partial signal PAZ to low level inresponse to the chip enable signal CE's turning to high level (FIG. 5(j)). The word line control circuit 34 changes the word line controlsignal WLZ to low level in response to the partial signal PAZ of lowlevel (FIG. 5( k)).

The word decoder WDEC releases the address signals A0X and A1X from thefixed high level in response to the partial signal PAZ of low level(FIG. 5( l)). The word decoder WDEC also changes the word line signalWLP to low level in response to the word line control signal WLZ of lowlevel (FIG. 5( m)). The word line signal WLP's changing to low levelcauses release of the connection between the individual memory cells MCin the partial area PA and the bit lines BL, so that the memory cells MCretain the data that has been retained before the shifting to thepartial mode. That is, the data of the memory cells MC in the partialarea PA is retained during the partial mode.

The core control circuit 28 changes the precharging signal PREZ to highlevel in response to the partial signal PAZ of low level (FIG. 5( n)).The change in the precharging signal PREZ precharges the bit lines BL(FIG. 5( o)). The sense amplifier control circuit 32 changes the senseamplifier activating signal LEZ to low level in response to the partialsignal PAZ (FIG. 5( p)). The change in the LEZ signal inactivates thesense amplifiers SA, so that the data latched in the sense amplifiers SAdisappears (FIG. 5( q)).

As above, according to the present embodiment, the data retained by thememory cells MC in the partial area PA is kept latched in the senseamplifiers SA during the partial mode. This can eliminate the need for arefresh operation to retain the data in the memory cells MC. The senseamplifiers are composed of CMOS circuits. Thus, the sense amplifiers SAare low in power consumption even if they keep latching data.Consequently, the power consumption during the partial mode can bereduced significantly as compared to heretofore.

During the partial mode, the word decoder WDEC disables the selection ofthe word lines WL other than in the partial area PA. Consequently, theword lines WL can be prevented from multiple selection in the partialmode, avoiding data crash.

While the sense amplifiers SA keep latching data during the partialmode, the word line control circuit 34 keeps selecting the partial wordline WLP. This simplifies the selecting/deselecting control of the wordlines, allowing a reduction in the scale of the word line controlcircuit 34.

In accordance with the chip enable signal CE for operating the pseudoSRAM, the operation mode shifts to the normal operation mode or thepartial mode. Thus, the operation mode of the pseudo SRAM can be shiftedby simple control. As a result, the control circuit of a systemimplementing the pseudo SRAM can be configured simply.

FIG. 6 shows a second embodiment of the semiconductor memory of thepresent invention. The same elements as those described in the firstembodiment will be designated by identical reference numbers or symbols.Detailed description thereof will be omitted here.

In this embodiment, a PA control circuit 12A is formed instead of the PAcontrol circuit 12 of the first embodiment. There is also provided abooster 36 for supplying the word lines WL with a boost voltage VPPwhich is higher than the power supply voltage. The rest of theconfiguration is almost the same as in the first embodiment. That is,the semiconductor memory of the present embodiment is formed as a pseudoSRAM with DRAM memory cells and an SRAM interface, by using CMOStechnology.

The PA control circuit 12A outputs the partial signal PAZ and a pulsesignal PAPZ. When the pulse signal PAPZ is at high level, the booster 36is activated to operate, generating the boost voltage VPP. The boostvoltage VPP is supplied to the word decoder WDEC.

FIG. 7 shows the details of the PA control circuit 12A and the word linecontrol circuit 34 shown in FIG. 6.

The PA control circuit 12A has an edge generating circuit 36 whichreceives the output of an AND circuit, and an OR circuit which receivesthe output of the edge generating circuit 36 and the output of the ANDcircuit. The edge generating circuit 36 outputs the pulse signal PAPZ ofhigh level in synchronization with the transition edges of the signalthat is output from the AND circuit. The OR circuit outputs, as thepartial signal PAZ, the OR logic between the inverted signal of the chipenable signal CE and the pulse signal PAPZ when the mode signal PAMDZ isat high level.

The word line control circuit 34 outputs the word line control signalWLZ of predetermined pulse width in synchronization with the RDZ signal,WRZ signal, or REFZ signal when the pulse signal PAPZ is at low level.In synchronization with the period where the pulse signal PAPZ is athigh level, the word line control circuit 34 changes the word linecontrol signal WLZ to high level.

FIG. 8 shows the operation of the pseudo SRAM of the second embodiment.Description will be omitted of the same operation as in the firstembodiment (FIG. 5). In this example, the mode register 14 shown in FIG.6 is set at the partial mode.

Initially, in the normal operation mode, the PA control circuit 12Ashown in FIG. 6 changes the pulse signal PAPZ to high level for apredetermined period and changes the partial signal PAZ to high level inresponse to the chip enable signal CE's turning to low level (FIG. 8(a)). The word line control circuit 34 changes the word line controlsignal WLZ to high level in response to the high-level period of thepulse signal PAPZ (FIG. 8( b)).

In response to the change in the word line control signal WLZ, the wordline WLP rises to the boost voltage (FIG. 8( c)). Then, as in FIG. 5,the data retained by the memory cells MC in the partial area PA is readto the bit lines BL and latched into the sense amplifiers SA (FIG. 8(d)).

Incidentally, the selection of the word lines WL by using the boostvoltage higher than the power supply voltage can lower the ON resistanceof the transfer transistors in the memory cells MC. This can increasethe charges to be retained by the memory cells MC, with extended refreshintervals in the normal operation mode.

Next, the word line control signal WLZ changes to low level in responseto the pulse signal PAPZ's changing to low level (FIG. 8( e)). The wordline signal WLP's changing to low level causes release of the connectionbetween the memory cells MC in the partial area PA and the bit lines BL.That is, the data retained by the memory cells MC in the partial area PAdisappears gradually. Moreover, after the pulse signal PAPZ changes tolow level, the booster 36 for generating the boost voltage to besupplied to the word lines WL stops operating. This allows a reductionin the power consumption of the booster 36 in the partial mode.

Meanwhile, the sense amplifier activating signal LEZ is kept at highlevel while the partial signal PAZ is at high level. Thus, the senseamplifiers SA keep retaining the data (FIG. 8( f)). Subsequently, thepulse signal PAPZ changes to high level again in synchronization withthe chip enable signal CE's changing to high level (FIG. 8( g)). Inresponse to the pulse signal PAPZ of high level, the word line controlsignal WLZ and the word line signal WLP change to high level insuccession (FIG. 8( h)). Then, the memory cells MC in the partial areaPA and the bit lines BL are connected with each other, so that the datalatched in the sense amplifiers SA is written to the memory cells MC.That is, the data having been written in the memory cells MC in thepartial area PA before the shifting to the partial mode is retainedwithout loss.

Subsequently, the partial signal PAZ changes to low level in response tothe pulse signal PAPZ's changing to low level (FIG. 8( i)). The changein the partial signal PAZ inactivates the sense amplifiers SA andprecharges the bit lines BL. The operation mode of the pseudo SRAM thusshifts from the partial mode to the normal operation mode.

As above, this embodiment can offer the same effects as those of thefirst embodiment described above. Besides, in this embodiment, the wordline WLP is selected for a predetermined period on shifting to thepartial mode so that the data retained by the memory cells MC is latchedinto the sense amplifiers SA. At the time of returning from the partialmode to the normal operation mode, the word line WLP is selected for apredetermined period again so that the data latched in the senseamplifiers SA is written to the memory cells MC. Since the word line WLPneed not be maintained at high level throughout the partial mode, powerconsumption can be reduced of the circuitry for generating the highlevel of the word line WLP. In particular, for the case of a pseudo SRAMin which the word lines WL are supplied with the boost voltage, thepower consumption of the booster 36 for generating the boost voltage canbe reduced in the partial mode. This consequently allows a furtherreduction in the power consumption of the pseudo SRAM in the partialmode.

FIG. 9 shows a third embodiment of the semiconductor memory of thepresent invention. The same elements as those described in the firstembodiment will be designated by identical reference numbers or symbols.Detailed description thereof will be omitted here.

The pseudo SRAM of this embodiment has a refresh timer 16B, a corecontrol circuit 28B, a word line control circuit 34B, and a memory core30B instead of the refresh timer 16, the core control circuit 28, theword line control circuit 34, and the memory core 30 of the firstembodiment. Moreover, a refresh register 38 is formed additionally. Therest of the configuration is almost the same as in the first embodiment.

The refresh register 38 receives the partial signal PAZ from the PAcontrol circuit 12 and the refresh request signal TREF from the refreshtimer 16B, and outputs refresh control signals REF1Z and REF2Z. Therefresh timer 16B receives the partial signal PAZ and the refreshcontrol signals REF1Z and REF2Z, and outputs the refresh request signalTREF.

The word line control circuit 34B of the core control circuit 28Breceives the read control signal RDZ, the write control signal WRZ, therefresh control signal REFZ, and the partial control signal PAZ, andoutputs word line control signals WLAZ and WLBZ. The memory core 30B hasa word decoder WDEC that is different from the one in the firstembodiment. The rest of the configuration is the same as in the firstembodiment. Like the first embodiment, the core control circuit 28Boperates as an operation control circuit for performing read operations,write operations, and refresh operations.

FIG. 10 shows the details of the refresh timer 16B shown in FIG. 9.

The refresh timer 16B includes an oscillating circuit 40 a consisting oftwo oscillators OSC1 connected in series, an oscillating circuit 40 bconsisting of a single oscillator OSC1, and a selector 40 c forselecting either of the outputs of the oscillating circuits 40 a and 40b and outputting it as the refresh request signal TREF. The threeoscillators OSC1 have the same oscillation cycle. When the refresh timer16B receives the PAZ signal of high level and the REF1Z signal and REF2Zsignal of low level, it outputs the refresh request signal TREF havingthe cycle of the oscillating circuit 40 a. When the refresh timer 16Breceives the PAZ signal of low level, the REF1Z signal of high level, orthe REF2Z signal of high level, it outputs the refresh request signalTREF having the cycle of the oscillating circuit 40 b.

FIGS. 11 and 12 show the details of the refresh register 38 shown inFIG. 9. FIG. 11 shows a circuit for generating the refresh controlsignal REF1Z, and FIG. 12 a circuit for generating the refresh controlsignal REF2Z. For ease of explanation, FIGS. 11 and 12 deal with thecase where the pseudo SRAM has eight word lines WL and all the memorycells MC are refreshed by eight refresh request signals TREF. In fact,the pseudo SRAM has 2048 word lines WL, for example. In this case, thenumbers of latches 38 a and 38 c shown in FIGS. 11 and 12 are eleveneach (11-bit counters).

In FIG. 11, the refresh register 38 has a latch 38 b. The latch 38 breceives the outputs of the respective latches 38 a constituting a 3-bitcounter and the output of the counter, and outputs the refresh controlsignal REFI Z when the partial signal PAZ is at high level. The latches38 a and 38 b are initialized in synchronization with the rising edge ofthe partial signal PAZ. The latches 38 a, when initialized, reset theirrespective output signals EXT1A, EXT2A, and EXT3A to low level.

The latch 38 a at the initial stage operates receiving the refreshrequest signal TREF at its clock terminal CK when the partial signal PAZis at high level. The second latch 38 a operates receiving the refreshrequest signal TREF at its clock terminal CK when the partial signal PAZand the output signal EXT1A are at high level. The third latch 38 aoperates receiving the refresh request signal TREF at its clock terminalCK when the partial signal PAZ and the output signals EXT1A, EXT2A areat high level. The latch 38 b operates receiving the refresh requestsignal TREF at its clock terminal CK when the partial signal PAZ and theoutput signals EXT1A, EXT2A, and EXT3A are at high level.

In FIG. 12, the refresh register 38 has a latch 38 d. The latch 38 dreceives the outputs of the respective latches 38 c constituting a 3-bitcounter and the output of the counter, and outputs the refresh controlsignal REF2Z when the partial signal PAZ is at low level. The latches 38c and 38 d are initialized in synchronization with the falling edge ofthe partial signal PAZ. The latches 38 c, when initialized, reset theirrespective output signals EXT1B, EXT2B, and EXT3B to low level.

The latch 38 c at the initial stage operates receiving the refreshrequest signal TREF at its clock terminal CK when the partial signal PAZis at low level. The second latch 38 c operates receiving the refreshrequest signal TREF at its clock terminal CK when the partial signal PAZis at low level and the output signal EXT1B is at high level. The thirdlatch 38 c operates receiving the refresh request signal TREF at itsclock terminal CK when the partial signal PAZ is at low level and theoutput signals EXT1B, EXT2B are at high level. The latch 38 d operatesreceiving the refresh request signal TREF at its clock terminal CK whenthe partial signal PAZ is at low level and the output signals EXT1B,EXT2B, and EXT3B are at high level.

FIG. 13 shows the operation of the refresh timer 16B and the refreshregister 38.

The refresh register 38 starts counting the refresh request signal TREFin synchronization with the rising edge of the partial signal PAZ, whichvaries in synchronization with the chip enable signal CE. The refreshregister 38 maintains the refresh control signal REF1Z at high levelwhile counting eight refresh request signals TREF.

Moreover, the refresh register 38 starts to count the refresh requestsignal TREF in synchronization with the falling edge of the partialsignal PAZ. The refresh register 38 maintains the refresh control signalREF2Z at high level while counting eight refresh request signals TREF.

The refresh timer 16B outputs the refresh request signal TREF with thecycle of the oscillator OSC1 when the partial signal PAZ is at low leveland when the refresh control signals REF1Z, REF2Z are at high level. Therefresh timer 16B outputs the refresh request signal TREF with the cycletwice that of the oscillator OSC1 when the partial signal PAZ is at highlevel and the refresh control signals REF1Z, REF2Z are at low level.

Consequently, at the start and end of the partial mode, the refreshrequest signal TREF is output with the same cycle as in the normaloperation mode. In the middle of the partial mode, the refresh requestsignal TREF is output with the cycle twice as much as in the normaloperation mode. In fact, the refresh control signals REF1Z and REF2Z aremaintained at high level while 2048 refresh request signals TREF areoutput.

FIG. 14 shows the details of the word line control circuit 34B shown inFIG. 9.

The word line control circuit 34B is formed by adding a new edgedetecting circuit 34 c to the edge detecting circuit 34 a of the wordline control circuit 34 in the first embodiment. When the edge detectingcircuit 34 c detects the transition edges of the RDZ signal, the WRZsignal, and the REFZ signal, it generates a detecting signal of smallerpulse width than the edge detecting circuit 34 a does. Then, the edgedetecting circuit 34 a outputs the word line control signal WLAZ ondetecting the transition edges of the RDZ signal, the WRZ signal, andthe REFZ signal. The edge detecting circuit 34 c outputs the word linecontrol signal WLBZ having the pulse width smaller than that of the wordline control signal WLAZ, on receiving the transition edges of the RDZsignal, the WRZ signal, and the REFZ signal. Besides, the word linecontrol signal WLBZ is generated after the word line control signalWLAZ.

FIG. 15 shows the details of the word decoder WDEC shown in FIG. 9. Forthe sake of plain explanation, FIG. 15 shows only part of a circuit thatcorresponds to two bits of row address signals A0Z and A1Z. In fact, theword decoder WDEC receives address signals for selecting all the wordlines WL in the memory core 30.

The word decoder WDEC has a gate circuit 42 a and a selector 42 b. Thegate circuit 42 a masks the address signal A0Z to output high level whenthe partial signal PAZ and the refresh control signal REF1Z are at highlevel. The selector 42 b selects the word line control signal WLBZ whenthe refresh control signal REF1Z is at high level, and selects the wordline control signal WLAZ when the refresh control signal REF1Z is at lowlevel.

Then, in the normal operation mode, any of the word lines (word linesignals) WL0A, WL0B, WL1A, and WL1B changes to high level according tothe address signals A0X and A1X. During the partial mode, the loweraddress signal A0X is masked so that two word lines (for example, WL0Aand WL0B) are selected according to the address signal A1X.

Furthermore, on shifting from the normal operation mode to the partialmode (PAZ, REF1Z=“H”), the word lines WL ending in “A” are supplied witha high-level pulse that has the same pulse width as that of the wordline control signal WLAZ. The word lines WL ending in “B” are suppliedwith a high-level pulse that has the same pulse width as that of theword line control signal WLBZ (a pulse width smaller than that of theword line control signal WLAZ). At the time of returning from thepartial mode to the normal operation mode (REF2Z=“H”), the lower addresssignal A0X is masked so that two word lines (for example, WL0A and WL0B)are selected according to the address signal A1X. The two word lines aresupplied with a high-level pulse having the same pulse width as that ofthe word line control signal WLAZ.

FIG. 16 shows the details of essential parts of the memory core 30Bshown in FIG. 9.

In this embodiment, partial areas PA are established for two word lineseach (for example, WL0A and WL0B). During the partial mode, data isretained by the memory cells MC (first memory cells) connected to theword lines WL ending in “A” (first word lines) and the memory cells MC(second memory cells) connected to the word lines WL ending in “B”(second word lines). That is, the data capacity available for retentionduring the partial mode is a half of the memory capacity of the memorycore 30B. The rest of the basic configuration is the same as that of thememory core 30 in the first embodiment.

FIG. 17 shows an overview of refresh operations in the pseudo SRAM ofthe third embodiment.

At the start of the partial mode, the word line control signal WLAZ isoutput, and then the word line control signal WLBZ is output (FIG. 17(a)). Thus, the word line WL0A is selected before the word line WL0B is.The data in the memory cells MC connected to the word line WL0A is thenread to the bit lines BL (FIG. 17( b)). After the word line WL0A isselected, the sense amplifiers SA start operating before the selectionof the word line WL0B. By this operation, the data in the memory cellsconnected to the word line WL0A can be surely transferred to the memorycells connected to the word line WL0B.

The data amplified by the sense amplifiers SA is written to the memorycells MC connected to the word line WL0B through the selection of theword line WL0B (FIG. 17( c)). This operation can be repeated so that thedata retained in a single memory cell is shared between two memory cellsMC (common refresh).

During the partial mode, the word lines WL0A and WL0B are simultaneouslyselected in synchronization with the word line control signal WLAZ (FIG.17( d)), whereby a refresh operation is performed on two memory cells MCat a time (partial refresh). The sense amplifiers SA start operatingafter the selection of the word lines WL0A and WL0B. At the end of thepartial mode, the word lines WL0A and WL0B are simultaneously selectedin synchronization with the word line control signal WLAZ (FIG. 17( e)),whereby a refresh operation is performed on two memory cells MC at atime (concentrated refresh). Subsequently, in the normal operation mode,each single word line WL is selected successively in synchronizationwith the word line control signal WLAZ. Refresh operations are performedon the memory cells MC connected to the word lines WL in succession.

FIG. 18 shows the operation of the pseudo SRAM of the third embodiment.With reference to FIG. 18, description will be given of features of thepresent invention, or refresh operations on shifting from the normaloperation mode to the partial mode, during the partial mode, and onshifting from the partial mode to the normal operation mode.

Initially, in the normal operation mode, the refresh request signal TREFis output at an oscillation cycle T of the oscillating circuit 40 bshown in FIG. 10 (FIG. 18( a)). In accordance with the refresh requestsignal TREF, the word lines WL are selected one by one so that refreshoperations are performed during intervals between read and writeoperations (FIG. 18( b)).

On shifting from the normal operation mode to the partial mode, therefresh register 38 shown in FIG. 11 changes the refresh control signalREF1Z to high level in synchronization with high-level changes of thepartial signal PAZ, until the refresh request signal TREF is output apredetermined number of times (FIG. 18( c)). In synchronization with therefresh request signal TREF, the word decoder WDEC shown in FIG. 15selects the word lines WL in two. Here, a word line WL ending in “B” isselected after a word line WL ending in “A”. Consequently, the data inthe memory cells MC connected to the word line WL ending in “A” iswritten to the memory cells MC connected to the word line WL ending in“B” (common refresh operation). That is, in the partial mode, the datais retained by every two memory cells MC in the partial area PA.

The common refresh operation is performed on all the partial areas PA insuccession with the same cycle as that of the refresh request signalTREF in normal operations. This prevents the data retained in the memorycells MC from disappearing while common refresh operations areperformed.

After the common refresh operations are completed of all the partialareas PA, the data is retained by using every two memory cells MC. Thus,the time for which data can be retained becomes twice longer than whendata is retained by using each single memory cell MC. In fact, the dataretention characteristic is the sum of the retention characteristics oftwo memory cells MC. The time for which data can be retained thusbecomes more than twice as much as when data is retained by using eachsingle memory cell MC.

During the partial mode, the refresh timer 16B outputs the refreshrequest signal TREF with the oscillation cycle 2T of the oscillatingcircuit 40 a (FIG. 18( d)). In response to the refresh request signalTREF, the two word lines WL in each partial area PA are selected at thesame time. A refresh operation is performed simultaneously on two memorycells MC with respect to each bit line BL (partial refresh operation).The simultaneous selection of two memory cells MC makes the signalquantities to be transmitted to the bit lines BL twice as much as innormal operations. Consequently, during the partial mode, the dataretained in the memory cells MC will not disappear even if the refreshinterval is rendered twice as much as in normal operations.Subsequently, partial refresh operations are performed successively inresponse to the refresh request signals TREF (FIG. 18( e)).

In the case of shifting from the partial mode to the normal operationmode, the chip enable signal CE of high level is supplied to change thepartial signal PAZ to low level (FIG. 18( f)). The refresh register 38shown in FIG. 12 changes the refresh control signal REF2Z to high levelin synchronization with the partial signal PAZ's changing to low level,until the refresh request signal TREF is output a predetermined numberof times (FIG. 18( g)). In synchronization with the refresh requestsignal TREF, the word decoder WDEC selects the word lines WL in two atthe same timing. Then, refresh operations are performed on the memorycells MC in all the partial areas PA (concentrated refresh operation).

The refresh control signal REF2Z changes to low level, at a point intime at which the operation mode of the pseudo SRAM shifts from thepartial mode to the normal operation mode (FIG. 18( h)). The refreshtimer 16B outputs the refresh request signal TREF with the oscillationcycle T of the oscillating circuit 40 b (FIG. 18( i)). In accordancewith the refresh request signal TREF, the word lines WL are selected oneby one so that refresh operations are performed during intervals betweenread and write operations (FIG. 18) j)).

As above, this embodiment can offer the same effects as those of thefirst embodiment described above. Besides, in this embodiment, dataretained in a single memory cell MC is retained by using a plurality ofmemory cells MC during the partial mode. This allows an increase in theretention time for which data can be retained. Consequently, in thepartial mode, the refresh interval can be made longer than in the normaloperation mode. The lower frequency of refresh operations can reduce thepower consumption during the partial mode.

In the first refresh operation under the partial mode (in commonrefresh), it is possible to prevent the data in the second memory cellsMC from being read first to destroy the data retained in the firstmemory cells MC. That is, the pseudo SRAM can be prevented frommalfunctioning.

In second and subsequent refresh operations on each partial area PA inthe partial mode, the first and second word lines WLA and WLB areselected simultaneously. The word line control circuit 34B can thus beconfigured simply.

In second and subsequent refresh operations in the partial mode, therefresh timer 16B outputs the refresh request signal TREF at intervalslonger than in the normal operation mode. This can lower the refreshfrequency during the partial mode, with a reduction in powerconsumption.

FIG. 19 shows a fourth embodiment of the semiconductor memory of thepresent invention. The same elements as those described in the first andthird embodiments will be designated by identical reference numbers orsymbols. Detailed description thereof will be omitted here.

In this embodiment, a refresh timer 16C is formed instead of the refreshtimer 16B of the third embodiment. The rest of the configuration isalmost the same as in the third embodiment. That is, the semiconductormemory of the present embodiment is formed as a pseudo SRAM with DRAMmemory cells and an SRAM interface, by using CMOS technology.

FIG. 20 shows the details of the refresh timer 16C shown in FIG. 19.

The refresh timer 16C has oscillating circuits 40 a, 40 b, and 40 d, anda selector 40 e. The oscillating circuits 40 a and 40 b are the same asin the third embodiment (FIG. 10). The oscillating circuit 40 d has anoscillator OSC2 which is shorter than the oscillator OSC1 in oscillationcycle. The oscillation cycle of the oscillator OSC2 is set atapproximately the same as the cycle time tRC in read operations.

The selector 40 e outputs the output of the oscillating circuit 40 d asthe refresh request signal TREF in returning from the partial mode tothe normal operation mode (the REF2Z signal=“H”).

FIG. 21 shows the operation of the pseudo SRAM of the fourth embodiment.

As compared to the third embodiment, this embodiment is significantlyreduced in the period of concentrated refresh operations in returningfrom the partial mode to the normal operation mode. The rest of thetiming is the same as in the third embodiment. During the concentratedrefresh operations, each single refresh operation is performed in thecycle time tRC (several tens of ns). On the contrary, normal refreshintervals are several tens of μs. The period of the concentrated refreshoperations in this embodiment can thus be reduced significantly ascompared to the period of the concentrated refresh operations in thethird embodiment.

As above, this embodiment can offer the same effects as those obtainedfrom the first and third embodiments described above. Moreover, in thisembodiment, the refresh timer 16C outputs the refresh request signalTREF at shorter intervals on shifting from the partial mode to thenormal operation mode than in the normal operation mode. This allowsquick return from the partial mode to the normal operation mode, with animprovement in the operation efficiency of the system implementing thepseudo SRAM.

FIG. 22 shows a fifth embodiment of the semiconductor memory of thepresent invention. The same elements as those described in the first andthird embodiments will be designated by identical reference numbers orsymbols. Detailed description thereof will be omitted here.

In this embodiment, a refresh timer 16D and a memory core 30D are formedinstead of the refresh timer 16B and the memory core 30B of the thirdembodiment. There is also provided the booster 36 of the secondembodiment. The rest of the configuration is almost the same as in thethird embodiment.

FIG. 23 shows the details of the refresh timer 16D shown in FIG. 22.

The refresh timer 16D has an oscillating circuit 40 f instead of theoscillating circuit 40 a in the refresh timer 16B (FIG. 10) of the thirdembodiment. The rest of the configuration is the same as that of therefresh timer 16B. The oscillating circuit 40 f consists of fouroscillators OSC1 connected in series, and outputs a signal having acycle four times longer than that of the oscillators OSC1.

The refresh timer 16D outputs the refresh request signal TREF having thecycle of the oscillating circuit 40 b in normal operation mode, at thestart of partial mode (in common refresh), and at the end of the partialmode (in concentrated refresh). During the partial mode (during partialrefresh), the refresh timer 16D outputs the refresh request signal TREFhaving the cycle of the oscillating circuit 40 f. Thus, the refreshinterval during the partial mode is four times longer than the refreshinterval in normal operations. This is twice as much as in the thirdembodiment.

FIG. 24 shows the details of essential parts of the memory core 30Dshown in FIG. 22.

The memory core 30D has a switch circuit 44 consisting of nMOStransistors. The switch circuit 44 is positioned to divide the bit linesBL (BL0, BL1, BL2, . . . , BLm) into two equal parts each. That is, eachbit line BL is divided into first and second bit lines across the switchcircuit 44. Then, the memory cells MC connected to the bit lines BL onthe side of the switch circuit 44 closer to the sense amplifiers SA(first bit lines) form a plurality of partial areas PA.

The switch circuit 44 is connected to the partial signal PAZ through aninverter that has the function of converting the voltage level. Theinverter changes a partial signal PAX to the boost voltage when thepartial signal PAZ is at low level, and changes the partial signal PAXto a ground voltage when the partial signal PAZ is at high level. Thus,the switch circuit 44 turns on in response to the partial signal PAZ oflow level, and turns off in response to the partial signal PAZ of highlevel. The inverter for generating the partial signal PAX and the PAcontrol circuit 12 shown in FIG. 22 operate the switch circuit 44 as aswitch control circuit that turns on in the normal operation mode andturns off in the partial mode.

As in the third embodiment (FIG. 16), the partial areas PA areestablished for two word lines each (for example, WL0A and WL0B). Theoperation of each partial area PA is almost the same as in the thirdembodiment. That is, in the partial mode, the memory cells MC connectedto the word lines WL ending in “A” retain data. The word decoder WDEC,in the partial mode, fixes the top single bit of address to high level.Consequently, only a half of the word lines WL lying closer to the senseamplifiers SA are selected successively in response to the refreshrequest signal TREF.

Since a half of the memory cells MC formed in the memory core 30D areassigned for the partial areas PA, the data capacity available forretention in the partial mode is a fourth of the memory capacity of thememory core 30D.

In this embodiment, the bit lines BL connected with the memory cells MCin the partial areas PA have a length half as much as in the thirdembodiment. Thus, the bit line capacitances also become a half. The dataretained by the memory cells MC is read out by sharing the chargesstored in the memory cells MC between the memory cell capacitances andthe bit line capacitances, and amplifying the charges on the bit lines.Consequently, reducing the bit line capacitances by half can double thecharges on the bit lines during read relatively. As a result, therefresh interval (the cycle of occurrence of the refresh request signalTREF) during the partial mode can be rendered twice as much as in thethird embodiment. Hence, the oscillation cycle of the oscillatingcircuit 40 f may be four times longer than that of the oscillators OSC1.

This embodiment can offer the same effects as those obtained from thefirst and third embodiments described above. Moreover, in thisembodiment, the refresh interval during the partial mode can be renderedtwice as much as in the third embodiment. This allows a furtherreduction in the power consumption during the partial mode.

FIG. 25 shows the memory core according to a sixth embodiment of thesemiconductor memory of the present invention. The same elements asthose described in the first embodiment will be designated by identicalreference numbers or symbols. Detailed description thereof will beomitted here.

In this embodiment, the memory core 30E has a bit line pair structure inwhich the memory cells MC are connected to complementary bit lines BLand /BL alternately. For example, to read data from the memory cells MCconnected to the bit lines BL, the bit lines /BL are supplied with areference voltage. The sense amplifiers SA are connected to the bitlines BL and /BL, and amplify the voltage differences between the bitlines BL and /BL differentially. The partial area PA is composed of thememory cells MC that are connected to a word line WLP which is theclosest to the sense amplifiers SA.

FIG. 26 shows the details of the sense amplifiers SA and the columnswitches CSW shown in FIG. 25.

A sense amplifier SA includes two CMOS inverters connected to each otherat their inputs and outputs, a pMOS transistor (pMOS switch) forconnecting the sources of the pMOS transistors of the CMOS inverters toa power supply line, and an nMOS transistor (nMOS switch) for connectingthe sources of the nMOS transistors of the CMOS inverters to a groundline. The inputs (or outputs) of the CMOS inverters are connected to thebit lines BL and /BL, respectively. The pMOS switch and the nMOS switchturn on when the sense amplifier activating signal LEZ is at high level,thereby activating the CMOS inverters. The activation of the CMOSinverters amplifies a voltage difference between the bit lines BL and/BL differentially.

A column switch CSW has nMOS transistors for connecting the bit linesBL, /BL and the data bus DB, /DB, respectively. The nMOS transistorsturn on when the column line signal CLZ is at high level.

FIG. 27 shows the operation of the pseudo SRAM of the sixth embodiment.Description will be omitted of the same operation as in the firstembodiment.

At an initial state, the bit lines BL and /BL are precharged to thereference voltage (FIG. 27( a)). On shifting from the normal operationmode to the partial mode, the data retained by the memory cells MC isread to the bit lines /BL in synchronization with the selection of theword line WLP (FIG. 27( b)). Subsequently, the sense amplifieractivating signal LEZ changes to high level so that voltage differencesbetween the bit lines BL and /BL are amplified (FIG. 27( c)). Then, thesense amplifiers SA latch the data retained by the memory cells MC inthe partial area PA. During the partial mode, the data that the senseamplifiers SA read from the memory cells MC in the partial area PA iskept latched for data retention (FIG. 27( d)).

At the time of returning from the partial mode to the normal operationmode, the word line WLP is deselected and the data is retained by thememory cells MC in the partial area PA (FIG. 27( e)). Next, the senseamplifier activating signal LEZ changes to low level so that the senseamplifiers SA are inactivated. The precharging signal PREZ changes tohigh level, and the bit lines BL and /BL are precharged to the referencevoltage (FIG. 27( f)).

This embodiment can offer the same effects as those of the firstembodiment described above. Besides, in this embodiment, the pseudo SRAMhaving a memory core of bit line pair structure can be reducedsignificantly in the power consumption during the partial mode ascompared to heretofore.

FIG. 28 shows a seventh embodiment of the semiconductor memory of thepresent invention. The same elements as those described in the firstembodiment will be designated by identical reference numbers or symbols.Detailed description thereof will be omitted here.

This semiconductor memory is formed as a pseudo SRAM with DRAM memorycells and an SRAM interface, by using CMOS technology.

The pseudo SRAM includes a command decoder 46, an operation mode controlcircuit 48, a refresh timer 50, a refresh command generator 52, arefresh address counter 54, an address buffer 22, a data input/outputbuffer 24, a multiplexer 26, a core control circuit 56, and a memorycore 58. The refresh timer 50, the refresh command generator 52, and therefresh address counter 54 operate as a refresh control circuit forrefreshing memory cells. In addition, the operation mode control circuit48, the refresh timer 50, the refresh address counter 54, and the corecontrol circuit 56 operate as an operation control circuit forgenerating the selecting timing of word lines WL. Furthermore, the corecontrol circuit 56 operates as an operation control circuit forperforming read operations, write operations, and refresh operations.

The command decoder 46 receives command signals (a chip enable signalCE, a write enable signal /WE, and an output enable signal /OE) throughexternal terminals, decodes the received commands, and outputs a readcontrol signal RDZ or a write control signal WRZ. The command decoder 46also outputs a partial mode starting signal PREFS (pulse signal) insynchronization with the falling edge of the CE signal, and outputs apartial mode releasing signal PREFR (pulse signal) in synchronizationwith the rising edge of the CE signal.

The operation mode control circuit 48 outputs mode signals MODE1, MODE2,MODE3, and MODE4 in accordance with the partial mode starting signalPREFS, the partial mode releasing signal PREFR, and the refresh controlsignal REFZ. The refresh timer 50 outputs the refresh request signalTREF having oscillation cycles corresponding to the mode signalsMODE1-4. The refresh address counter 54 updates the refresh addresssignal REFAD (R5-0) in synchronization with the refresh control signalREFZ. The update specification of the refresh address signal REFAD ismodified in accordance with the mode signals MODE2-4.

The number of bits of the refresh address signal REFAD corresponds tothe number of word lines WL (64, in this example) formed in the memorycore 58. Hence, the number of bits of the refresh address signal REFADis not limited to 6 bits but set in accordance with the number of wordlines WL formed in the memory core 58.

The core control circuit 56 includes a timing control circuit 60, asense amplifier control circuit 62, and a precharge control circuit 64.The timing control circuit 60 outputs a row activating signal RASZ whenit receives any of the RDZ signal, the WRZ signal, and the REFZ signal.The sense amplifier control circuit 62 outputs the sense amplifieractivating signals PSA and NSA for activating sense amplifiers SA insynchronization with the RASZ signal. The precharge control circuit 64outputs the precharging signal PREZ in synchronization with the RASZsignal. The operation timing of the sense amplifier control circuit 62and the precharge control circuit 64 is changed in accordance with themode signal MODE2 and the least significant bit X0 of the refreshaddress signal REFAD.

The memory core 58 includes the sense amplifiers SA, a precharge circuitPRE, a memory cell array ARY, a word decoder WDEC, a column decoderCDEC, a sense buffer SB, and a write amplifier WA. The sense amplifiersSA operate in accordance with the sense amplifier activating signals PSAand NSA. The precharge circuit PRE operates in accordance with theprecharging signal PREZ. The memory cell array ARY has a plurality ofvolatile memory cells MC (dynamic memory cells), along with a pluralityof word lines WL and a plurality of bit lines BL connected to the memorycells MC. The memory cells MC are the same as typical DRAM memory cells,each having a capacitor for retaining data in the form of a charge and atransfer transistor arranged between this capacitor and a bit line BL.The gates of the transfer transistors are connected to the word linesWL.

The word decoder WDEC selects one or two of the word lines WL accordingto the row address signal RAD2 and the mode signals MODE3-4, and booststhe selected word line(s) WL to a power supply voltage. The columndecoder CDEC, the sense buffer SB, and the write amplifier WA are thesame circuits as in the first embodiment.

FIG. 29 shows the details of the operation mode control circuit 48 shownin FIG. 28.

The operation mode control circuit 48 has a counter 48 a and a modesignal generator 48 b. The counter 48 a counts in synchronization withthe rising edge of the refresh control signal REFZ. The counter 48 aoutputs a counter signal CNT32 at the 32nd count and a counter signalCNT64 at the 64th count. The counter 48 a is reset in response to areset signal RESET. The reset signal RESET is output when the modesignal MODE1 or the mode signal MODE3 is at high level.

Note that the count “64” corresponds to the number of word lines WLformed in the memory core 58. For plain explanation, the presentembodiment is given 64 word lines WL. In fact, there are provided 2048word lines WL, for example. In this case, the counter 48 a outputscounter signals at the 1024th count and the 2048th count, respectively.

The mode signal generator 48 b outputs the mode signals MODE1-4according to the partial mode starting signal PREFS, the partial modereleasing signal PREFR, and the counter signals CNT32 and CNT64.

FIG. 30 shows the operation of the operation mode control circuit 48shown in FIG. 28.

As in the third embodiment described above, the pseudo SRAM of thisembodiment enters the normal operation mode when the CE signal is athigh level, and enters the partial mode (low power consumption mode)when the CE signal is at low level. Then, at the start of the partialmode, common refresh is performed (common refresh mode). After thecommon refresh, partial refresh is performed (partial refresh mode). Atthe end of the partial mode, concentrated refresh is performed(concentrated refresh mode). During the normal operation mode, a singlememory cell is refreshed each time a sense amplifier SA operates (singlecell operation). During the low power consumption mode, two memory cellsare refreshed each time a sense amplifier SA operates (twin celloperation).

The pseudo SRAM recognizes the normal operation mode (first operationmode) when the mode signal MODE1 is at high level, recognizes the commonrefresh mode (second operation mode) when the mode signal MODE2 is athigh level, recognizes the partial refresh mode (third operation mode)when the mode signal MODE3 is at high level, and recognizes theconcentrated refresh mode (fourth operation mode) when the mode signalMODE4 is at high level.

When the operation mode control circuit 48 receives the partial modesetting signal PREFS during the normal operation mode, it changes themode signals MODE1 and MODE2 to low level and high level, respectively.The operation mode thus shifts from the normal operation mode to thecommon refresh mode (partial mode) (FIG. 30( a)). The reset signal RESETis inactivated in synchronization with the mode signal MODE1's changingto low level.

In response to the low level of the reset signal RESET, the counter 48 ais released from a reset state, and starts to count in synchronizationwith the refresh control signal REFZ (FIG. 30( b)). Refresh operationsare performed in response to the refresh control signal REFZ. Since allthe word lines WL in the memory core 58 must be selected in the commonrefresh mode, the refresh control signal REFZ is output 64 times.Incidentally, the operation of the refresh timer 50 and the refreshcommand generator 52 for generating the refresh control signal REFZ willbe described in FIG. 32 to be seen later.

The counter 48 a outputs the counter signal CNT64 in synchronizationwith the 64th count operation (FIG. 30( c)). In synchronization with thecounter signal CNT64, the operation mode control circuit 48 changes themode signal MODE2 to low level and changes the mode signal MODE3 to highlevel (FIG. 30( d)). Then, the operation mode shifts from the commonrefresh mode to the partial refresh mode. The reset signal RESET isactivated in synchronization with the mode signal MODE3's changing tohigh level (FIG. 30( e)). The counter 48 a is reset in response to thehigh level of the reset signal RESET. While the mode signal MODE3 is athigh level, partial refresh is performed successively.

The partial mode releasing signal PREFR is output in response to the CEsignal's changing to high level supplied through the external terminal(FIG. 30( f)). When the operation mode control circuit 48 receives thepartial mode releasing signal PREFR during the partial refresh mode, itchanges the mode signals MODE3 and MODE4 to low level and high level,respectively. The operation mode thus shifts to the concentrated refreshmode (FIG. 30( g)). The reset signal RESET is inactivated insynchronization with the mode signal MODE3's turning to low level. Inresponse to the low level of the reset signal RESET, the counter 48 a isreleased from the reset state, and starts counting again insynchronization with the refresh control signal REFZ (FIG. 30( h)).

In the concentrated refresh mode, two word lines WL (a partial word lineand a normal word line adjacent to this partial word line) are selectedat the same time. To select all the word lines WL in the memory core 58,the refresh control signal REFZ is output 32 times.

The counter 48 a outputs the counter signal CNT32 in synchronizationwith the 32nd count operation (FIG. 30( i)). In synchronization with thecounter signal CNT32, the operation mode control circuit 48 changes themode signals MODE4 and MODE1 to low level and high level, respectively(FIG. 30( j)). Then, the operation mode shifts from the concentratedrefresh mode (partial mode) to the normal operation mode.

FIG. 31 shows the details of the refresh timer 50 shown in FIG. 28.

The refresh timer 50 includes an oscillator 50 a for generating anoscillation signal OSC0, frequency dividers 50 b, 50 c, 50 d, and 50 efor dividing the OSC0 signal in frequency to generate oscillationsignals OSC1, OSC2, OSC3, and OSC4, respectively, and a multiplexer 50 ffor selecting the oscillation signals OSC1, OSC2, OSC3, and OSC4according to the mode signals MODE1-4 and outputting the resultant asthe refresh request signal TREF. The frequency dividers 50 b, 50 c, 50d, and 50 e convert the OSC0 signal to ⅛, 1/16, 1/32, and ½ infrequency, respectively.

FIG. 32 shows the operation of the refresh timer 50 and the refreshcommand generator 52.

The refresh timer 50 outputs the oscillation signals OSC1, OSC2, OSC3,and OSC4 as the refresh request signal TREF when the mode signals MODE1,MODE2, MODE3, and MODE4 are at high level, respectively. The refreshcommand generator 52 outputs the refresh request signal TREF as therefresh control signal REFZ when the mode signals MODE1, MODE3, andMODE4 are at high level, respectively. When the mode signal MODE2 is athigh level, the refresh command generator 52 outputs the refresh controlsignal REFZ twice in synchronization with the refresh request signalTREF.

FIG. 33 shows the details of the refresh address counter 54 shown inFIG. 28.

The refresh address counter 54 has a resetting circuit 54 a, counters 54b and 54 c, and logic gates for controlling the counters 54 b and 54 c.The resetting circuit 54 a includes a pulse generator for generating apositive pulse in synchronization with the falling edge of the refreshcontrol signal REFZ, a D flip-flop for latching the mode signal MODE2 insynchronization with the output signal of the pulse generator, and aNAND gate for detecting the rising edge of the mode signal MODE2.

The counter 54 b counts in synchronization with the refresh controlsignal REFZ to generate the least significant bit R0 of the refreshaddress signal REFAD. The counter 54 b is reset when the mode signalMODE3 or MODE4 is at high level, or in synchronization with the risingedge of the mode signal MODE2.

The counter 54 c counts in synchronization with the refresh controlsignal REFZ to update the bits R5-1 of the refresh address signal REFADwhen the mode signal MODE3 or MODE4 is at high level. When the modesignal MODE1 or MODE2 is at high level (except in a predetermined periodafter the rising edge of the mode signal MODE2), the counter 54 c countsin synchronization with the address signal R0 which is output from thecounter 54 b, thereby updating the bits R5-1.

FIG. 34 shows the operation of the resetting circuit 54 a shown in FIG.33.

The pulse generator outputs a pulse signal to the node ND1 insynchronization with the falling edge of the refresh control signal REFZ(FIG. 34( a)). The D flip-flop latches the mode signal MODE2 insynchronization with the pulse signal on the node ND1, and outputs theinverted logic of the mode signal MODE2 to the node ND2 (FIG. 34( b)).Consequently, the node ND2 changes to low level in synchronization withthe first refresh control signal REFZ after the mode signal MODE2changes to high level(FIG. 34( c)). Then, the AND logic of the logiclevels of the mode signal MODE2 and the node ND2 is output to the nodeND3. The counter 54 b shown in FIG. 33 is reset while the node ND3 is athigh level, i.e., in the first refresh operation period after the modesignal MODE2 changes to high level.

FIG. 35 shows the operation of the refresh address counter 54 shown inFIG. 33.

The refresh address counter 54 successively counts up the 6-bit refreshaddress signal R5-0 in synchronization with the refresh control signalREFZ when the mode signal MODE1 or MODE2 is at high level, i.e., in thenormal operation mode (first operation mode) and in the common refreshmode (second operation mode). Besides, the refresh address counter 54successively counts up the 5-bit refresh address signal R5-1 insynchronization with the refresh control signal REFZ when the modesignal MODE3 or MODE4 is at high level, i.e., in the partial mode (thirdoperation mode) and in the concentrated refresh mode (fourth operationmode). Here, the refresh address signal R0 is fixed to low level.

FIG. 36 shows the details of essential parts of the memory core 58 shownin FIG. 28.

The word decoder WDEC of the memory core 58 has a ¼ word decoder 59 anda plurality of sub word decoders 58 a corresponding to main word linesMW (MW0, MW1, . . . ), respectively. The ¼ word decoder 59 outputs anyof decoding signals X11, X10, X01, and X00 according to the lower twobits X1 and X0 of the row address signal RAD2 and the inverted bits /X1and /X0 thereof when the mode signals MODE3 and MODE4 are at low level.When either of the mode signals MODE3 and MODE4 is at high level, the ¼word decoder 59 outputs two decoding signals X11 and X10, or X0 and X00according to the lower one bit X1 of the row address signal RAD2 and theinverted bit /X1 thereof.

The sub word decoders 58 a are activated when the respective main wordlines MW are at high level, selecting a sub word line SW (SW0P, SW1,SW2P, SW3, . . . ) according to the decoding signals X11, X10, X01, andX00. The main word lines MW are selected by a not-shown predecoder, inaccordance with the upper bits of the row address signal RAD2. Then, thememory cells MC connected to the selected sub word line SW are accessed.As above, in this embodiment, the word lines WL shown in FIG. 28 consistof the main word lines MW and the sub word lines SW.

The sub word lines SW ending in “P” represent partial word lines. Duringthe partial mode, data written in the memory cells MC connected to thepartial word lines SWP (partial memory cells C00) is retained. The subword lines SW with no last “P” represent common word lines. Data in thememory cells MC connected to the common word lines SW (common memorycells C10) will not be retained during the partial mode.

The partial word lines SWP and the normal sub word lines SW are laidalternately. That is, the word lines SWP and SW are arranged adjacent toeach other. As will be described later, during the partial mode, theword lines SWP and SW are selected in synchronization with each other sothat two memory cells are accessed at a time (twin cell operation).Consequently, the adjacent, alternate arrangement of these word linesSWP and SW prevents the word decoder WDEC from having intricate wiringlayout therein. In particular, it is easy to design the wiring layout ofthe sub word decoders 58 a.

In this embodiment, a half of the memory cells MC formed in the memorycore 58 are partial memory cells. That is, data as much as a half of thememory capacity of the pseudo SRAM is retained during the partial mode.

FIG. 37 shows the details of the ¼ word decoder 59 shown in FIG. 36.

The ¼ word decoder 59 has a decoder 59 a and a mask circuit 59 b. Thedecoder 59 a decodes the row address signals X0, /X0, X1, and /X1 togenerate the decoding signals X11, X10, X0, and X00. The mask circuit 59b masks the row address signals X0 and /X0 to output high level to thedecoder 59 a when the mode signal MODE3 or MODE4 is at high level.

FIG. 38 shows the details of the sense amplifiers SA and the prechargecircuits PRE shown in FIG. 36.

The sense amplifiers SA are the same as those of the sixth embodiment(FIG. 26) except in that the pMOS switch and the nMOS switch arecontrolled by the sense amplifier activating signals PSA and NSA,respectively. The pMOS switch turns on when the sense amplifieractivating signal PSA is at low level. The nMOS switch turns on when thesense amplifier activating signal NSA is at high level.

Each precharge circuit PRE includes an nMOS transistor for connectingthe bit lines BL and /BL to each other, and nMOS transistors forconnecting the bit lines BL, /BL to a precharge voltage line VPR,respectively. The nMOS transistors turn on when the precharging signalPREZ is at high level, thereby connecting the bit lines BL, /BL to theprecharge voltage line VPR.

FIG. 39 shows the operation of the sense amplifier control circuit 62and the precharge control circuit 64 shown in FIG. 28.

Regardless of the logic level of the mode signal MODE2, the senseamplifier control circuit 62 changes the sense amplifier activatingsignals PSA and NSA a delay time DLY1 after the rising edge of the RASZsignal, thereby activating the sense amplifiers SA (FIG. 39( a, b)).Regardless of the logic level of the mode signal MODE2, the prechargecontrol circuit 64 changes the precharging signal PREZ to low level insynchronization with the rising edge of the RASZ signal, therebystopping a precharge operation (FIG. 39( c, d)).

When the mode signal MODE2 is at low level (when in the first operationmode, the third operation mode, and the fourth operation mode), thesense amplifier control circuit 62 changes the sense amplifieractivating signals PSA and NSA a delay time DLY2 after the rising edgeof the RASZ signal, thereby inactivating the sense amplifiers SA (FIG.39( e)). When the mode signal MODE2 is at low level, the prechargecontrol circuit 64 changes the precharging signal PREZ to high level thedelay time DLY2 after the rising edge of the RASZ signal, therebystarting a precharge operation (FIG. 39( f)).

When the mode signal MODE2 is at high level (when in the secondoperation mode), the sense amplifier control circuit 62 changes thesense amplifier activating signals PSA and NSA the delay time DLY2 afterthe rising edge of the RASZ signal, after the row address signal X0changes to high level. This inactivates the sense amplifiers SA (FIG.39( g)). When the mode signal MODE2 is at high level, the prechargecontrol circuit 64 changes the precharging signal PREZ to high level thedelay time DLY2 after the rising edge of the RASZ signal, after the rowaddress signal X0 changes to high level, thereby starting a prechargeoperation (FIG. 39( h)).

That is, during the second operation mode (common refresh mode), inorder that the data retained by the partial memory cells C00 be writtento the partial memory cells and the adjacent common memory cells C10,the sense amplifiers SA are activated and precharging of the bit linesBL, /BL is inhibited while the RASZ signal is output twice. Morespecifically, the data latched into the sense amplifiers SA insynchronization with the refresh control signal REFZ that is outputunder the row address signal X0 of an even number is retained until anoperation corresponding to the refresh control signal REFZ that isoutput after the row address signal X0 changes to an odd number.

FIG. 40 shows the operation of the seventh embodiment in the normaloperation mode.

Among the commands CMD to be issued to operate the pseudo SRAM in thenormal operation mode are a read command and a write command suppliedthrough external terminals, and a refresh command (REFZ signal) from therefresh command generator 52.

For example, the partial memory cells C00 are accessed by the firstcommand CMD, and the common memory cells C10 are accessed by the nextcommand CMD. The word lines SW0P and SW1 are selected independentlyaccording to the row address signal RAD2.

When the command signals CMD are read commands, the data amplified onthe bit lines BL and /BL is output to exterior through the data bus DB.When the commands CMD are write commands, data supplied through theexternal terminal is amplified by the write amplifier WA and the senseamplifiers SA, and written to the memory cells. When the commands CMDare refresh commands, data amplified by the sense amplifiers SA isrewritten to the memory cells.

FIG. 41 shows the operation of the seventh embodiment in the commonrefresh mode.

In the common refresh mode, the partial memory cells C00 are initiallyaccessed so that data retained by the partial memory cells C00 islatched into the sense amplifiers SA (FIG. 41 (a)). Next, with the senseamplifiers SA activated, the common memory cells C10 are accessed sothat the data (complementary data) latched in the sense amplifiers SA iswritten to the common memory cells C10 (FIG. 41( b)). Consequently, thepartial memory cells C00 and the common memory cells C10 retain mutuallycomplementary data. Then, the foregoing operation is performed on allthe partial areas PA.

FIG. 42 shows the operation of the seventh embodiment in the partialrefresh mode and the concentrated refresh mode.

In the partial refresh mode and the concentrated refresh mode, thepartial word line SW0P and the common word line SW1 are selected at thesame time. The complementary data retained in the partial memory cellsC00 and the common memory cells C10 is simultaneously amplified by thesense amplifiers SA and rewritten to the cells C00 and C10 (twin celloperation). Since the data is retained by using the partial memory cellsC00 and the common memory cells C10, the refresh interval can beextended significantly.

In the partial refresh mode, the charge retained by each single memorycell immediately before a refresh operation is smaller than in thenormal operation mode, as much as the refresh interval is extended.Consequently, in case of a direct shift from the partial refresh mode tothe normal operation mode, memory cells on which a long time passesafter refresh operations have been performed may suffer a failure indata read (data crash). Thus, before the shifting to the normaloperation mode, refresh operations are performed on all the partialmemory cells C00 in the concentrated refresh mode. The concentratedrefresh mode requires only that data be rewritten to the partial memorycells C00. The refresh interval may thus be shorter than in the normaloperation mode. In this embodiment, as shown in FIG. 32, the refreshinterval is rendered ¼ the refresh interval in the normal operationmode. The refresh interval in the concentrated refresh mode may be thesame as the read operation cycle tRC in the normal operation mode.

FIG. 43 shows the operation of the pseudo SRAM of the seventhembodiment. The timing chart shown to the bottom in the diagram followsthe timing chart shown to the top in the diagram.

In the normal operation mode, each single sub word line SW is selectedin response to the refresh control signal REFZ (single cell operations).When the CE signal changes to low level for shifting from the normaloperation mode to the common refresh mode, the resetting circuit 54 a ofthe refresh address counter 54 shown in FIG. 33 resets the counter 54 bfor generating the least significant bit X0 of the row address signalRAD2, in synchronization with the rising edge of the mode signal MODE2so that the partial word lines SWP are selected initially.

After the partial word lines SWP are all selected, the operation modeshifts from the common refresh mode to the partial refresh mode. In thepartial refresh mode, twin cell operations (refresh operations) areperformed in which two adjacent sub word lines SW are selected for eachsingle refresh control signal REFZ.

When the CE signal changes to high level during the partial refreshmode, the operation mode shifts to the concentrated refresh mode. In theconcentrated refresh mode, twin cell operations are performed at shorterrefresh intervals. Then, after the twin cell operations are performed onall the sub word lines SW, the operation mode shifts to the normaloperation mode.

As above, the present embodiment can offer the same effects as those ofthe third embodiment described above. Besides, in this embodiment, thepartial word lines SWP and the common word lines SW in the respectivesame partial areas PA are arranged adjacent to each other. This cansimplify the circuit layout of the word decoder WDEC for selecting theword lines SWP and SW.

In accordance with the chip enable signal CE for operating the pseudoSRAM, the operation mode shifts to the normal operation mode or thepartial mode. Thus, the operation mode of the pseudo SRAM can be shiftedby simple control. As a result, the control circuit of the systemimplementing the pseudo SRAM can be configured simply.

FIG. 44 shows an eighth embodiment of the semiconductor memory of thepresent invention. The same elements as those described in the first andseventh embodiments will be designated by identical reference numbers orsymbols. Detailed description thereof will be omitted here.

In this embodiment, an operation mode control circuit 66, a refreshtimer 68, a refresh command generator 52A, a refresh address counter 70,a core control circuit 56A, and a memory core 58A are formed instead ofthe operation mode control circuit 48, the refresh timer 50, the refreshcommand generator 52, the refresh address counter 54, the core controlcircuit 56, and the memory core 58 of the seventh embodiment. The restof the configuration is almost the same as in the seventh embodiment.The operation mode control circuit 66, the refresh timer 68, the refreshaddress counter 70, and the core control circuit 56A operate as anoperation control circuit.

The sense amplifier control circuit 62A and the precharge controlcircuit 64A receive the lower two bits X1 and X0 of the row addresssignal RAD output from the multiplexer 26.

FIG. 45 shows the details of the operation mode control circuit 66 shownin FIG. 44.

The operation mode control circuit 66 has a counter 66 a and a modesignal generator 66 b. The counter 66 a counts in synchronization withthe rising edge of the refresh control signal REFZ. The counter 66 aoutputs a counter signal CNT16 at the 16th count and a counter signalCNT64 at the 64th count.

FIG. 46 shows the operation of the operation mode control circuit 66shown in FIG. 45.

In this embodiment, the refresh control signal REFZ is output four timesin succession during the common refresh mode. In the partial refreshmode and the concentrated refresh mode, partial memory cells C00connected to a single partial word line SWP to be described later andcommon memory cells C10, C20, and C30 connected to three common wordlines SW, respectively, are refreshed at a time (quad cell operation).Thus, during the concentrated refresh mode, 16 refresh control signalsREFZ are output to refresh the entire retained data.

FIG. 47 shows the details of the refresh timer 68 shown in FIG. 44.

The refresh timer 68 has frequency dividers 50 b, 50 c, 68 a, and 50 ewhich convert the OSC0 signal to ⅛, 1/16, 1/64, and ½ in frequency,respectively.

FIG. 48 shows the operation of the refresh timer 68 and the refreshcommand generator 52A.

The refresh timer 68 outputs the oscillation signals OSC1, OSC2, OSC3,and OSC4 as the refresh request signal TREF when the mode signals MODE1,MODE2, MODE3, and MODE4 are at high level, respectively. The refreshcommand generator 52A outputs the refresh request signal TREF as therefresh control signal REFZ when the mode signals MODE1, MODE3, andMODE4 are at high level, respectively. When the mode signal MODE2 is athigh level, the refresh command generator 52A outputs the refreshcontrol signal REFZ four times in synchronization with the refreshrequest signal TREF.

FIG. 49 shows the details of the refresh address counter 70 shown inFIG. 44.

The refresh address counter 70 has a resetting circuit 54 a, counters 70a and 70 b, and logic gates for controlling the counters 70 a and 70 b.The counter 70 a counts in synchronization with the refresh controlsignal REFZ to generate the lower two bits R1 and R0 of the refreshaddress signal REFAD. The counter 70 a is reset when the mode signalMODE3 or MODE4 is at high level, or in synchronization with the risingedge of the mode signal MODE2.

When the mode signal MODE3 or MODE4 is at high level, the counter 70 bcounts in synchronization with the refresh control signal REFZ to updatethe upper four bits R5-2 of the refresh address signal REFAD. When themode signal MODE1 or MODE2 is at high level (except in a predeterminedperiod after the rising edge of the mode signal MODE2), the counter 70 bcounts in synchronization with the address signal R1 output from thecounter 70 a, thereby updating the bits R5-2.

FIG. 50 shows the operation of the refresh address counter 70 shown inFIG. 49.

When the mode signal MODE1 or MODE2 is at high level, the refreshaddress counter 70 successively counts up the 6-bit refresh addresssignal R5-0 in synchronization with the refresh control signal REFZ.When the mode signal MODE3 or MODE4 is at high level, the refreshaddress counter 70 successively counts up the 4-bit refresh addresssignal R5-2 in synchronization with the refresh control signal REFZ.Here, the refresh address signal R1, R0 is fixed to low level.

FIG. 51 shows the details of essential parts of the memory core 58Ashown in FIG. 44.

The word decoder WDEC of the memory core 58A has a ¼ word decoder 72 anda plurality of sub word decoders 58 b corresponding to main word linesMW (MW0, MW1, . . . ), respectively. The ¼ word decoder 72 outputs anyof decoding signals X11, X10, X01, and X00 according to the lower twobits X1 and X0 of the row address signal RAD2 and the inverted bits /X1and /X0 thereof when the mode signals MODE3 and MODE4 are at low level.The ¼ word decoder 72 changes all the decoding signals X11, X10, X01,and X00 to high level when either of the mode signals MODE3 and MODE4 isat high level.

In this embodiment, the memory cells (C00, C10, C20, C30, . . . )connected to adjacent four sub word lines (for example, SW0P, SW1, SW2,and SW3) form a partial area PA. For example, the sub word line SW0P isa partial word line connected to the partial memory cells C00 whose datais retained in the partial mode. The sub word lines SW1, SW2, and SW3are common word lines connected to the common memory cells C10, C20, andC30 whose data will not be retained in the partial mode.

The partial memory cells C00 and the common memory cells C20 areconnected to the bit lines BL, and the common memory cells C10 and C30are connected to the bit lines /BL. In the partial mode, the partialword line SW0P and the common word lines SW1, SW2, and SW3 are selectedin synchronization with one another so that four memory cells areaccessed at a time (quad cell operation). Then, the data retained by thepartial memory cell C00 during the normal operation mode is retained bythe four memory cells C00, C10, C20, and C30 during the partial mode.

In this embodiment, a quarter of the memory cells MC formed in thememory core 58A are partial memory cells. That is, data as much as aquarter of the memory capacity of the pseudo SRAM is retained during thepartial mode.

FIG. 52 shows the details of the ¼ word decoder 72 shown in FIG. 51.

The ¼ word decoder 72 has a decoder 72 a and a mask circuit 72 b. Thedecoder 72 a decodes the row address signals X0, /X0, X1, and /X1 togenerate the decoding signals X11, X10, X01, and X00. The mask circuit72 b masks the row address signals X0, /X0, X1, and /X1 to output highlevel to the decoder 72 a when the mode signal MODE3 or MODE4 is at highlevel.

FIG. 53 shows the operation of the sense amplifier control circuit 62Aand the precharge control circuit 64A shown in FIG. 44. For situationswhere the mode signal MODE2 is at low level and where the mode signalMODE2 changes to high level, the operation is the same as in the seventhembodiment (FIG. 39).

When the mode signal MODE2 is at high level (when in the secondoperation mode), the sense amplifier control circuit 62A changes thesense amplifier activating signals PSA and NSA a delay time DLY2 afterthe rising edge of the RASZ signal, after the row address signals X1 andX0 both change to high level. This inactivates the sense amplifiers SA(FIG. 53( a)). When the mode signal MODE2 is at high level, theprecharge control circuit 64A changes the precharging signal PREZ tohigh level the delay time DLY2 after the rising edge of the RASZ signal,after the row address signals X1 and X0 both change to high level,thereby starting a precharge operation (FIG. 53( b)).

That is, during the second operation mode (common refresh mode), inorder that the data retained by the partial memory cells C00 be writtento the partial memory cells and the adjacent common memory cells C10,C20, and C30, the sense amplifiers SA are activated and the bit linesBL, /BL are kept from being precharged while the RASZ signal is outputfour times.

FIG. 54 shows the operation of the eighth embodiment in the normaloperation mode.

In the normal operation mode, the word lines SW0P, SW1, SW2, and SW3 areselected independently according to the row address signal RAD2 as inthe seventh embodiment (FIG. 40). Then, in response to read commands orwrite commands from exterior, read operations or write operations areperformed. Refresh operations are performed in response to refreshcommands generated inside the pseudo SRAM.

FIG. 55 shows the operation of the eighth embodiment in the commonrefresh mode.

In the common refresh mode, the data retained by the partial memorycells C00 are initially latched into the sense amplifiers SA (FIG. 55(a)). Next, with the sense amplifiers SA activated, the common memorycells C10, C20, and C30 are accessed in succession so that the data(complementary data) latched in the sense amplifiers SA is written tothese common memory cells C10, C20, and C30 (FIG. 55( b, c, d)).Consequently, the partial memory cells C00 and the common memory cellsC10, C20, and C30 retain mutually complementary data. The foregoingoperation is performed on all the partial areas PA.

FIG. 56 shows the operation of the eighth embodiment in the partialrefresh mode and the concentrated refresh mode.

In the partial refresh mode and the concentrated refresh mode, thepartial word line SW0P and the common word lines SW1, SW2, and SW3 areselected at the same time. The complementary data retained in thepartial memory cells C00 and the common memory cells C10, C20, and C30is simultaneously amplified by the sense amplifiers SA and rewritten tothe cells C00, C10, C20, and C30 (quad cell operation). Since the datais retained by the partial memory cells C00 and the common memory cellsC10, C20, and C30, the refresh interval can be extended more than in theseventh embodiment.

The concentrated refresh mode, as in the seventh embodiment, is anoperation necessary to prevent the data retained by the partial memorycells C00 from disappearing upon shifting from the partial mode to thenormal operation mode.

As above, the present embodiment can offer the same effects as thoseobtained from the third and seventh embodiments described above.Moreover, in this embodiment, data retained in a single partial memorycell C00 is retained by using the partial memory cell C00 and commonmemory cells C10, C20, and C30 in the partial mode. This allows afurther increase in retention time over which the data can be retained.Consequently, the frequency of refresh operations can be reduced furtherwith a significant reduction in the power consumption during the partialmode.

Incidentally, the embodiments described above have dealt with the caseswhere the present invention is applied to a pseudo SRAM. However, thepresent invention is not limited to such embodiments. For example, thepresent invention may be applied to a DRAM that has a self refreshfunction.

The foregoing embodiments have dealt with the cases where the CE signal,the /WE signal, and the /OE signal are used as the command signals.However, the present invention is not limited to such embodiments. Forexample, as with a DRAM, a row address strobe signal /RAS and a columnaddress strobe signal /CAS may be used as the command signals.

The foregoing embodiments have dealt with the cases where the operationmode is changed to the partial mode when the chip enable signal CE is atlow level. However, the present invention is not limited to suchembodiments. For example, two chip enable signals /CE1 and CE2 may bereceived through external terminals. Here, normal read operations andwrite operations are enabled when the /CE1 signal is at low level andthe CE2 signal at high level, and the operation mode is changed to thepartial mode when the CE2 signal is at low level.

The first through fifth embodiments described above have dealt with thecases where the present invention is applied to a memory core having anarchitecture in which the sense amplifiers SA are connected to a singlebit line BL each. However, the present invention is not limited to suchembodiments. For example, the memory cores described in the firstthrough fifth embodiments may be replaced with a memory core that hascomplementary bit lines BL and /BL such as illustrated in the sixthembodiment.

The foregoing first through fifth embodiments have dealt with the caseswhere the mode register 14 is set with a predetermined value so that thelow power consumption mode is established as the partial mode where themode signal PAMDZ of high level is output. However, the presentinvention is not limited to such embodiments. For example, the moderegister may be made of a fuse circuit having a fuse with such aspecification that the partial mode for outputting the mode signal PAMDZis established when the fuse is blown in the fabrication process.Alternatively, the output of the fuse circuit may be input to the moderegister so that the mode register is set in accordance with theprogrammed state of the fuse upon power-up of the pseudo SRAM.Furthermore, the output level of the mode signal PAMDZ may be setaccording to the voltage value of a conductive layer that is formed onthe chip as corresponding to the pattern shape of a photomask used inthe fabrication process.

The third embodiment (FIG. 18) and fourth embodiment (FIG. 21) describedabove have dealt with the cases where concentrated refresh operationsare performed on the memory cells MC in all the partial areas PA.However, the present invention is not limited to such embodiments. Forexample, concentrated refresh operations may be performed only on memorycells MC on which a predetermined period elapses since previous refreshoperations have been performed. Here, an example of the predetermineperiod is the time equivalent to the refresh interval for a singlememory cell. In this case, the partial areas PA to undergo concentratedrefresh can be reduced approximately by half, with a reduction in theperiod required for concentrated refresh operations. The reduced numberof refresh operations can also reduce the power consumption.

The fifth embodiment described above has dealt with the case where theswitch circuit 44 is composed of nMOS transistors. However, the presentinvention is not limited to such an embodiment. For example, as shown inFIG. 57, the switch circuit 44 may be composed of CMOS transmissiongates. In this case, the bit lines BL can be reduced in resistance sothat refresh operations, read operations, and write operations areperformed at high speed.

The foregoing fifth embodiment has dealt with the case where the bitlines BL are divided into two equal parts to form the partial areas PA.However, the present invention is not limited to such an embodiment. Forexample, the bit lines BL may be divided into four equal parts to formthe partial areas PA. Here, the refresh intervals can be rendered fourtimes longer, with a further reduction in the power consumption duringthe partial mode.

The sixth embodiment described above has dealt with the case where theword line WLP is continuously selected during the partial mode. However,the present invention is not limited to such an embodiment. For example,as in the second embodiment, the word line WLP may be selected only atthe start and at the end of the partial mode. In particular, when theword lines are supplied with a boost voltage, the frequency of operationof the booster can be lowered during the partial mode with a furtherreduction in power consumption.

The seventh embodiment (FIG. 41) described above has dealt with the casewhere the partial word line SW0P and the common word line SW1 aresuccessively selected in the common refresh mode. However, the presentinvention is not limited to such an embodiment. For example, as shown inFIG. 58, the partial word line SW0P may be kept selected until it isdeselected along with the common word line SW1. In this case, the wordlines can be deselected by a single operation of the resetting circuitwith a reduction in power consumption. The operation shown in FIG. 58may also be applied to the eighth embodiment (FIG. 55).

The foregoing eighth embodiment (FIG. 56) has dealt with the case wherethe partial word line SW0P and the common word lines SW1, SW2, and SW3are selected simultaneously during the partial refresh mode and duringthe concentrated refresh mode. However, the present invention is notlimited to such an embodiment. For example, as shown in FIG. 59, theselection starting timing of the word lines SW0P, SW1, SW2, and SW3 canshift in succession to suppress power supply noise. In particular, whenthe timing specification shown in FIG. 59 is applied to a pseudo SRAM inwhich a boost voltage is supplied to the word lines, the consumptioncurrent can be dispersed to lower the capacity of the booster. As aresult, the power consumption can be reduced further to reduce the powersupply noise that occurs with the operation of the booster.

The invention is not limited to the above embodiments and variousmodifications may be made without departing from the spirit and scope ofthe invention. Any improvement may be made in part or all of thecomponents.

1. A semiconductor memory comprising: a plurality of memory cells; a bitline connected to said memory cells; a sense amplifier connected to saidbit line; a partial area composed of a first memory cell of said memorycells; an operation control circuit for operating any of said memorycells selected in accordance with an address signal, during normaloperation mode in which a read operation and a write operation areperformed, and for keeping latching data retained by said first memorycell into said sense amplifier, during low power consumption mode inwhich data only in said first memory cell is retained; a plurality ofword lines connected to said memory cells, respectively, said word linesbeing selected in accordance with said address signal, wherein saidoperation control circuit further comprises: a word line control circuitfor selecting any of said word lines in accordance with said addresssignal during said normal operation mode, and for enabling selection ofa partial word line which is one of said word lines and disablingselection of the other word lines during said low power consumptionmode, said partial word line being connected to said first memory cell;and a sense amplifier control circuit for keeping activating said senseamplifier during said low power consumption mode; and further whereinsaid word line control circuit keeps selecting said partial word lineduring said low power consumption mode; and a booster for supplying aboost voltage to said work lines, wherein at the start of said low powerconsumption mode, said booster stops its boosting operation after saidsense amplifier latches data.
 2. The semiconductor memory according toclaim 1, wherein said booster restarts its boosting operation to supplysaid boost voltage to said partial word line, in returning from said lowpower consumption mode to said normal operation mode.
 3. Thesemiconductor memory according to claim 1, wherein said word linecontrol circuit selects said partial word line for a predeterminedperiod to allow said sense amplifier to latch data, in shifting fromsaid normal operation mode to said low power consumption mode.
 4. Thesemiconductor memory according to claim 1, wherein said word linecontrol circuit selects said partial word line for a predeterminedperiod to write data latched in said sense amplifier to said firstmemory cell, in returning from said low power consumption mode to saidnormal operation mode.
 5. A semiconductor memory comprising: a pluralityof memory cells; a bit line connected to said memory cells; a senseamplifier connected to said bit line; a refresh control circuit forcyclically outputting a refresh control signal for refreshing saidmemory cells; an operation control circuit for performing a readoperation, a write operation, and a refresh operation on said memorycells; and a plurality of partial areas each composed of predeterminednumbers of said memory cells connected to said bit line, wherein: saidpartial areas each include a single first memory cell and at least asingle second memory cell both of which are among said memory cellsconnected to said bit line; and said operation control circuit performs,at the start of low power consumption mode, a refresh operation in whichdata retained in said first memory cell is amplified by said senseamplifier and written to said first and second memory cells, andsubsequently refreshes said first and second memory cells simultaneouslyin response to said refresh control signal during said low powerconsumption mode.
 6. The semiconductor memory according to claim 5,comprising a first word line connected to said first memory cell andsecond word line(s) connected to said second memory cell(s) in each ofsaid partial areas, wherein said operation control circuit includes aword line control circuit for starting selection of said first word lineearlier than selection of said second word line(s) during a firstrefresh operation on each of said partial areas in said low powerconsumption mode.
 7. The semiconductor memory according to claim 6,wherein said word line control circuit selects said first and secondword lines simultaneously during second and subsequent refreshoperations on each of said partial areas in said low power consumptionmode.
 8. The semiconductor memory according to claim 6, wherein saidoperation control circuit includes a sense amplifier control circuit foroutputting a sense amplifier activating signal between the selection ofsaid first word line and the selection of said second word line duringthe first refresh operation on each of said partial areas in said lowpower consumption mode, the sense amplifier activating signal activatingsaid sense amplifier.
 9. The semiconductor memory according to claim 8,wherein said sense amplifier control circuit outputs said senseamplifier activating signal after the start of selection of said firstand second word lines, during second and subsequent refresh operationson each of said partial areas in said low power consumption mode. 10.The semiconductor memory according to claim 5, wherein during a firstrefresh operation on each of said partial areas in said low powerconsumption mode, said refresh control circuit outputs said refreshcontrol signal at the same intervals as in normal operation mode. 11.The semiconductor memory according to claim 10, wherein during secondand subsequent refresh operations on each of said partial areas in saidlow power consumption mode, said refresh control circuit outputs saidrefresh control signal at intervals longer than in said normal operationmode.
 12. The semiconductor memory according to claim 5, wherein inshifting from said low power consumption mode to normal operation mode,said refresh control circuit outputs said refresh control signal at thesame intervals as in said normal operation mode.
 13. The semiconductormemory according to claim 12, wherein in shifting from said low powerconsumption mode to said normal operation mode, said refresh controlcircuit performs a refresh operation only on memory cell(s), of saidmemory cells, on which a predetermined length of time elapses after aprevious refresh operation has been performed.
 14. The semiconductormemory according to claim 13, wherein said predetermined length of timeis equal to a refresh interval for each of said memory cells during saidnormal operation mode.
 15. The semiconductor memory according to claim5, wherein in shifting from said low power consumption mode to normaloperation mode, said refresh control circuit outputs said refreshcontrol signal at intervals shorter than in normal operation mode so asto perform a concentrated refresh operation.
 16. The semiconductormemory according to claim 15, wherein in shifting from said low powerconsumption mode to said normal operation mode, said refresh controlcircuit performs a refresh operation only on memory cell(s), of saidmemory cells, on which a predetermined length of time elapses after aprevious refresh operation has been performed.
 17. The semiconductormemory according to claim 16, wherein said predetermined length of timeis equal to a refresh interval for each of said memory cells during anormal operation.
 18. A semiconductor memory comprising: a plurality ofmemory cells; a bit line connected to said memory cells; a senseamplifier connected to said bit line; a refresh control circuit forcyclically outputting a refresh control signal for refreshing saidmemory cells; a switch circuit for dividing said bit line into first andsecond bit lines; a partial area composed of first memory cell(s) ofsaid memory cells, said first memory cell(s) being connected to saidfirst bit line lying on a side of said switch circuit closer to saidsense amplifier; and a switch control circuit for turning on said switchcircuit in normal operation mode and turning off the same in low powerconsumption mode.
 19. The semiconductor memory according to claim 18,comprising: a plurality of word lines connected to said memory cells,respectively, said word lines being selected in accordance with anaddress signal; and a word line control circuit for selecting any ofsaid word lines in accordance with said address signal during saidnormal operation mode, and enabling selection of partial word line(s)and disabling selection of the other word lines during said low powerconsumption mode, said partial word line(s) being among said word linesand connected to said first memory cell(s) in said partial area.
 20. Thesemiconductor memory according to claim 19, wherein said word linecontrol circuit selects said partial word lines in succession in saidlow power consumption mode.
 21. The semiconductor memory according toclaim 18, wherein: said switch circuit is made of an nMOS transistor;and a gate of said nMOS transistor receives a high level voltage to turnon in said normal operation mode and receive a low level voltage to turnoff in said low power consumption mode.
 22. The semiconductor memoryaccording to claim 19, wherein said high level voltage is a boostvoltage higher than a power supply voltage.
 23. The semiconductor memoryaccording to claim 18, wherein said switch circuit is made of a CMOStransmission gate.